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ELEMENTAKY PHYSICAL GEOGKAPHY 



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Plate 1. — Frontispiece. 
Watkins Glen, N7¥. A post-glacial gorge in a shale rock. 



ELEMENTARY 



PHYSICAL GEOGRAPHY 



BY 



RALPH S. TARR, B.S., F.G.S.A. 

PROFESSOR OF DYNAMIC GEOLOGY AND PHYSICAL GEOGRAPHY 

AT CORNELL UNIVERSITY 

AUTHOR OF " ECONOMIC GEOLOGY OF THE UNITED STATES " 



THE MACMILLAN COMPANY 

LONDON: MACMILLAN & CO., Ltd. 
1907 

All rights reserved, 



T\JL 

SUIERARYo. CONGRESS 
{ tfne «uuv Kfcceivea 

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COPY A. 






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Copyright, 1895, 
By MACMILLAN AND CO. 



Set up and electrotyped October, 1895. Reprinted March, 
July, October, 1896; May, July, September, 1897; July, 1898; 
August, 1899; August, 1900 ; April, December, 1902; Novem- 
ber, 1903 ; April, 1905 ; October, 1907. 



PREFACE. 

For some time there have been indications that new text- 
books on physical geography are demanded ; and in the 
report of the Committee of Ten this finds definite expression. 
In the preparation of this book, which has been in hand for 
several years, there is an attempt to meet this apparent 
demand; but for reasons which are obvious to many, it has 
not seemed wise to attempt to follow the somewhat radical 
suggestions which were made by the majority of the geogra- 
phy conference of the Committee of Ten. Therefore, while 
the physiographic side is given more prominence than is 
customary in works of this kind, this book attempts to only 
partly meet the Committee's suggestions. 

In the preparation of the book, effort has been made to 
introduce new material, particularly in the illustrations, 
which are a prominent part of the book. Also, there has 
been an endeavor to make the book scientifically accurate, 
and to introduce the latest knowledge on the subjects 
treated. There are probably places in which this is not 
done, for the field is so large that much must be compilation ; 
and the compiler is liable to fall into error. 

I anticipate criticism of the order of presentation, of the 
relative amount of space allotted the various topics, of the 



viii PREFACE. 

omission of some subjects which are usually found in such 
books, and of the inclusion of some not usually discussed ; 
but these matters have been carefully considered, and the 
book is the result of a well-matured plan. In many respects 
it is experimental, but it is a deliberate attempt to supply a 
book which is certainly needed. It should not be inferred 
that the author is satisfied with the attempt, — he is keenly 
disappointed at the constant necessity of saving space and 
thereby weakening description and explanation. In many 
cases, explanations have been omitted; in others, perhaps it 
would have been better to have done so. 

It is hoped that the more advanced teachers will find it 
possible to accompany the text-book work with laboratory 
and field study, along the line suggested in the appendix. 
The discussion of method has been systematically eliminated 
from the text, and the sole effort has been to present facts 
and furnish information ; but if this alone is put before the 
pupils, the value of the study will be very slight indeed. It 
furnishes the main story in a connected way, and supplies 
certain information ; but the laboratory and field will supply 
applications and extensions of the principles, at the same 
time giving value to the study as a means of mental disci- 
pline. Merely to hear recitations from the book, will be the 
continuation of an all too prevalent habit, which in so many 
cases makes the science teaching in our secondary schools 
the weakest part of the curriculum. 

While the author has done much work in some of the 
subjects treated, particularly the ocean and the land, he 
would not wish to claim that much in the book is original. 
In reality, this book is based upon the manuscript of another 
and more advanced work, which is soon to be published as 



PREFACE. ix 

a handbook for teachers and for reference. Both of these 
represent an attempt to gather from all available sources, the 
kind of matter which it seemed desirable to include in such 
books. While in the larger work direct reference is made 
to the sources of information, it has not seemed desirable to 
do so in this case ; for the acknowledgments take much space 
and distract the attention, without benefiting the pupil. 

I have had much generous assistance in the supply of illus- 
trations, particularly of photographs ; and grateful general 
acknowledgment is made here, while special mention of the 
sources is made in a list in the succeeding pages. Although 
I have received aid from many sources, there are a few which 
I must mention especially. The writings of Geikie, Dutton, 
Powell, and Gilbert, particularly the latter, have not only 
given me bodies of fact, but also inspiration, as indeed they 
have to all who are working in physiographic geology. To 
the writings and teachings of Professors Shaler and Davis 
of Harvard University, I owe more than I could possibly 
acknowledge ; and to the latter I am under an added obliga- 
tion for his examination and kindly criticism of parts of my 
manuscript. While I acknowledge the debt which I owe 
these scientists, it must be understood that the mode of 
presentation is my own, and that I alone am responsible for 
any shortcomings which may appear. 

RALPH S. TARR. 
Ithaca, N.Y., August 30, 1895. 



CONTENTS. 

Part I. The Air. 
CHAPTER I. The Earth as a Planet. 

PAGE 

Form of the Earth 3 

The Solar System 5 

The Sun 6 

The Planets 8 

Asteroids 11 

The Earth 11 

The Moon 13 

Comets, Shooting- Stars, and Meteors .15 

The Stellar System 17 

Symmetry of the Solar System 18 

The Nebular Hypothesis 19 

Verification of the Nebular Hypothesis 20 

CHAPTER II. The Atmosphere. 

General Statement 23 

Light 25 

Electricity and Magnetism 29 

Heat 30 

Moisture 35 

Pressure 39 

Effect of Gravity 39 

Effect of the Earth's Rotation . .39 

CHAPTER III. Distribution of Temperature. 

General Statement 43 

Effect of Atmospheric Movements 44 

xi 



Xli CONTENTS. 

PAGE 

Influence of Oceans 45 

Effect of Topography .47 

Seasonal Temperature Range .48 

Isothermal Charts ........... 51 

Daily Temperature Curve .60 

Temperature Ranges 62 

Earth Temperatures 65 



CHAPTER IV. General Circulation of the Atmosphere. 

General Statement 68 

Classification of the Winds 70 

Planetary or Permanent Winds 71 

Trade Winds .71 

Doldrum Belt 74 

Anti-trade Winds 74 

Horse Latitude Winds 75 

Prevailing Westerlies 75 

Periodical Winds 76 

Seasonal Winds 76 

Migrating Wind and Calm Belts 76 

Monsoon Winds 77 

Diurnal Winds 79 

Sea and Land Breezes 79 

Mountain and Valley Breezes 80 

Eclipse and Tidal Breezes 82 

Irregular Winds 82 

Accidental Winds .......... 82 

The Nature of Winds 83 



CHAPTER V. Storms. 

Cyclonic Storms 85 

Hurricanes 86 

Description 86 

Effects 88 

Path 90 

Time of Occurrence 91 

Cause 91 



CONTENTS. 



Xlll 



Temperate Latitude Cyclones . 

Resemblance to Hurricanes 

Differences from Hurricanes 

Effects 

Winds 

Anticyclones 

Cause 
Secondary Storms . 
Thunderstorms 
Tornadoes and Waterspouts 



PAGE 

93 

93 

95 

98 

98 

100 

100 

101 

101 

104 



CHAPTER VI. The Moisture of the Atmosphere. 



Dew . 107 

Frost . . . . .108 

Fog 109 

Haze . 110 

Mist Ill 

Clouds Ill 

Rain 114 

Snow 115 

Hail 116 

Distribution of Rainfall in the World . . . . . . . 117 

Distribution of Rainfall in the United States 118 

Distribution of Snowfall ......... 121 

Seasonal Distribution of Rainfall ........ 122 

Irregularities of Rainfall 123 

CHAPTER VII. Weather and Climate. 

Weather 124 

Tropical and Arctic 124 

Temperate Latitude Weather 125 

Climate 129 

Tropical Climate 130 

Temperate Climate .......... 130 

Arctic Climate 132 

Minor Variations 132 

Changes in Climate 132 



xiv CONTENTS. 

CHAPTER VIII. Geographic Distribution of Animals 
and Plants. 

PAGE 

General Statement 135 

The Ocean 135 

FreshWater 137 

The Land 137 

Effect of Temperature and Moisture . . . . . . 137 

Plant and Animal Habits 141 

Life Zones ' . 143 

The Spread of Life 145 

Barriers to the Spread of Life 147 

Effect of Man 147 



Part II. The Ocean. 



CHAPTER IX. Form and General Characteristics of 
the Ocean. 

Distribution of Land and Water 151 

Composition of Ocean Water 151 

Color and Phosphorescence 152 

Exploration of the Ocean Bottom 153 

Methods used in Deep-sea Explorations 153 

Sounding . 153 

Dredging 155 

Topography of the Ocean Bottom 156 

General 156 

The Atlantic Ocean 158 

Other Oceans 160 

Topography near the Coast 160 

Temperature of the Ocean Bottom 162 

Light on the Ocean Bottom 163 

Materials composing the Ocean Floor 164 

Mechanical Sediments 164 

Globigerina Ooze 164 

Red Clay 165 

Life in the Ocean 166 

Pelagic or Surface Faunas 166 

Littoral or Shore Faunas 167 

Faunas of the Ocean Bottom 169 



CONTENTS. XV 



CHAPTER X. Ocean Waves and Currents. 

PAGK 

Wind Waves 174 

Earthquake Waves 178 

Storm Waves 179 

Ocean Surface Temperatures 179 

Ocean Currents 182 

Planetary Circulation . . . . . . . . 182 

The System of Ocean Currents 183 

Cause of Ocean Currents 185 

The Gulf Stream 187 

The Labrador Current 189 

Effects of Ocean Currents . . . . . . . . 189 

CHAPTER XI. Tides. 

Nature of the Tidal Wave 192 

Cause of Tides 192 

Effect of the Land 193 

Other Causes for Variation in Tidal Height 198 

Effects of Tides * . 201 



Part III. The Land. 
CHAPTER XII. The Crust of the Earth. 

Interior Condition 205 

Movements of the Crust 206 

Disturbance of the Rocks . 207 

Volcanic Action 211 

Rocks of the Earth's Crust 212 

Igneous Rocks 213 

Metamorphic Rocks 214 

Sedimentary Rocks 214 

Deposition of Sedimentary Rocks 215 

Consolidation of Sedimentary Rocks ....... 217 

Geological Chronology 218 

Age of the Earth 221 



XVI CONTENTS. 



CHAPTER XIII. Denudation of the Land. 

PAGE 

Underground Water . 224 

The Formation of Caverns 226 

Springs and Artesian Wells 228 

Durability of Rocks 231 

Weathering 233 

Agents of Erosion 238 

Wind Erosion 238 

Rain Erosion 239 

Percolating Water 240 

River Erosion 241 

Ocean Erosion ........... 244 

Glacial Erosion 245 

Denudation 246 



CHAPTER XIV. Topographic Features of the Earth's 

Surface. 

Continents and Ocean Basins 249 

Physical Geography of the United States 253 

Atlantic Coast Area ......... 254 

The Eastern Mountains 254 

The Canadian Highlands 256 

The Central Plains ' . . 256 

The Cordilleran Area 25 

The Drainage of the Country 259 

The Shore Line 26i 



CHAPTER XV. River Valleys. 

General Description 262 

Development of River Valleys ........ 265 

Adjustment of Streams 272 

The River Divide 273 

Accidents to Streams . . 275 

Land Movements 276 

Climatic Accidents .......... 279 

Other Accidents -..2 



CONTENTS. XVll 



CHAPTER XVI. Deltas, Floodplains, Waterfalls, 
and Lakes. 

PAGE 

Deltas . . 285 

Floodplains 288 

Waterfalls ' 294 

Lakes 298 

Swamps 303 

CHAPTER XVII. Glaciers. 

Cause of Glaciers ........... 306 

Alpine or Valley Glacier ......... 307 

Continental Glaciers 313 

Icebergs 315 

Glacial Period ........... 316 

Area covered by Ice ......... 316 

Terminal Moraine .......... 319 

Formation of Soil .......... 321 

Formation of Lakes 323 

Formation of Waterfalls 325 

v/cHAPTER XVIII. The Coast Line. 

General Statement 328 

Effect of Elevation . 329 

Effect of Depression 329 

.Effect of Sediment 330 

.Effect of Waves and Currents 332 

Effect of Plants 337 

Effect of Animals . 340 

Changes in Coast Form 343 

Islands . . . . 344 

Promontories 346 

Lake Shores . . . . . .347 

Fossil Shore Lines 348 

^CHAPTER XIX. Plateaus and Mountains. 

Plateaus 350 

Mountains 353 

Characteristics of Mountains 353 



XV111 CONTENTS. 

PAGB 

The Origin of Mountains 362 

Sculpturing of Mountains . 364 

The Drainage of Mountains ........ 365 

Destruction of Mountains 367 

CHAPTER XX. Volcanoes, Earthquakes, and Getsees. 

Volcanoes 370 

Distribution 370 

Materials Erupted .371 

Eruptions of Volcanoes 374 

Eorm of Cone 378 

Effects of Volcanic Eruptions 381 

Extinct Volcanoes 381 

Cause of Volcanoes 383 

Earthquakes 383 

Geysers and Hot Springs 386 

CHAPTER XXI. The Topography of the Land. 

General Statement 390 

Constructive Land Forms 392 

By Internal Forces 392 

By Agents of Denudation 393 

By Animal and Plant Life 395 

Effect of Rock Structure upon Topography 395 

CHAPTER XXII. Man and Nature. 

General Statement 407 

Modifying Influence of Man 407 

Man and the Forest 409 

Influence of Nature upon Man 412 

CHAPTER XXIII. Economic Products of the Earth. 

Soil 420 

Building Stones 420 

Economic Deposits of Sedimentary Origin 422 

Miscellaneous Substances 423 



CONTENTS. xix 

PAGE 

Coal 423 

Natural Gas and Petroleum . 425 

Ore Deposits 426 

Distribution of Ore Deposits 428 

Mineral Wealth of the United States 429 



APPENDIX I. 
Meteorological Instruments, Apparatus, and Methods. 

Thermometric Records 431 

Barometric Records 432 

Measurement of Wind Direction and Force 433 

Measurement of Evaporation . . . 434 

Measurement of Moisture in the Air 434 

Study of Clouds and Sunshine 434 

Measurement of Rainfall 435 

Meteorological Methods and Results 435 



APPENDIX II. 
Topographic Maps 437 

APPENDIX III. 
Suggestions to Teachers 440 

APPENDIX IV. 
Questions upon the Text 453 



ILLUSTRATIONS. 



DIAGRAMS AND PHOTOGRAPHS. 

FIG. PAGE 

1. Sphere and oblate spheroid 3 

2. Land and water hemispheres 4 

3. The solar system 5 

4. Relative size of sun and large planets 7 

5. Sun spots, 1872 8 

6. Relative distances of planets from the sun 8 

7. Relative size of smaller planets 9 

8. Illustration of the cause of seasons 12 

9. Relative size of earth and moon 14 

10. Lunar craters 14 

11. Comet of Donati, 1858 15 

12. Orbit of comet of 1862 16 

13. Andromeda nebula 17 

14. Thickness of the atmosphere 23 

15. Decrease in density of the atmosphere .24 

16. Passage of sun's rays through the atmosphere .... 26 

17. Inclination of the sun's rays 34 

18. Daily change in relative humidity . . 37 

19. Increase in temperature of descending air 38 

20. Deflection of air currents 40 

21. Decrease in diameter on different latitudes 40 

22. Daily temperature curves 44 

23. Irregularities of seasonal curve . . . . . . .45 

24. Seasonal temperature ranges 49 

25. Seasonal temperature range (New York) 51 

26. Isotherms for February (northern hemisphere) .... 52 

27. Daily temperature curve (summer and winter) .... 60 

28. Daily temperature range for several days 61 

29. Daily temperature record for several days ..... 61 

xxi 



xxil ILL US TEA TIONS. 

FIG. PAGE 

30. Temperature ranges, United States, 1892 62 

31. Minimum temperatures, United States, 1892 63 

32. Maximum temperatures, United States, 1892 64 

33. Daily temperature range near and above the ground ... 66 

34. General circulation of the globe 69 

35. Summer monsoons, India 77 

36. Winter monsoons, India 77 

37. The sea breeze 78 

38. The land breeze . 79 

39. Effect of sea breeze on air temperature 80 

40. Valley breeze 81 

41. Ideal diagram of a storm 85 

42. Barometric record during passage of a hurricane . .- .86 

43. Diagram of hurricane winds . . . . . . .87 

44. Map of a hurricane .88 

45. Tracks of August hurricanes 89 

46. Map of temperate latitude cyclone 94 

47. Paths of low-pressure areas .95 

48. Average storm tracks, 1878-1887 (Northern hemisphere) . . 96 

49. Tracks of low-pressure areas .97 

50. Photograph of thunderstorm . . . 102 

51. Path of thunderstorm . 103 

52. View of a tornado 104 

53. Effect of tornado at Lawrence, Mass 105 

54. Distribution of tornadoes (1794-1881) . . . . . .106 

55. Valley fog in the Himalayas 110 

56. The banner cloud Ill 

57. Photographs of clouds 112 

58. Photographs of snowflakes 115 

59. Damp snowfall 116 

60. Evaporation in United States 120 

61. Monthly rainfall in the West .121 

62. Variation in annual rainfall in the West 122 

63. A cold wave .127 

64. Temperature descent during cold wave 128 

65. Climatic zones ........... 129 

66. Near the timber line 138 

67. Above the snow line, Mount St. Elias, Alaska .... 139 

68. Effect of sunlight on mountain vegetation . . . . . 140 

69. Arid land vegetation 141 

70. Arid land vegetation, Eio Grande valley 142 



ILLUSTBATIONS. xxiii 

FIO. PAGE 

71. The tropical forest 143 

72. Life zones of United States 144 

73. Deep-sea sounding machine 154 

74. Deep-sea trawl 155 

75. Contrast between land and ocean bottom topography . . . 156 

76. Cross-section of Atlantic Ocean 158 

77. Temperature of the Mediterranean 163 

78. Globigerina ooze 165 

79. Coral reef on Australian coast 168 

80. Ocean waves 174 

81. Breakers on the coast 175 

82. Effect of storm waves on the coast 177 

83. Normal vertical descent of ocean temperatures .... 180 

84. Tides near Hell Gate, N.Y 196 

85. Time and height of tides at Hell Gate 196 

86. The tides at Eastport, Me., September, 1893 199 

87. Height of high tide, Eastport, Me., 1893 and 1894 . . .200 

88. Tidal mud flat in Bay of Fundy 202 

89. Tidal rise and fall, Cape Ann, Mass 203 

90. Horizontal rocks in Kansas 208 

91. A monoclinal fold .208 

92. Anticline 208 

93. Syncline . . . 209 

94. Photograph of anticline, Hancock, W.Va. . . . . 209 

95. Photograph of anticline near Quebec, Canada .... 209 

96. Photograph of a fault in Arizona 210 

97. Photograph of a fault in glacial clay, Massachusetts . . . 210 

98. A dike crossing granite 212 

99. Contorted limestone 214 

100. Stratified shale, near Ithaca, N.Y 215 

101. Section of alternating strata 216 

102. Unconformity in horizontal rocks 217 

103. Unconformity in inclined rocks 217 

104. Photograph of fossiliferous rock 219 

105. Mammoth Hot Springs, Yellowstone Park 225 

106. Diagram illustrating formation of caverns 226 

107. A sink hole in limestone region 227 

108. Stalactites in Luray Cave 227 

109. The Natural Bridge, Va 228 

110. A spring on a fault plane 228 

111. Hillside spring 229 



xxiv ILLUSTRATIONS. 

FIG. PAGE 

112. Photograph of an artesian well 229 

113. Artesian well in monoclinal strata 230 

114. Artesian well in syncline 230 

115. Rock pillars in Garden of Gods, Col 231 

116. The weathering of granite 233 

117. Effect of roots in breaking up rocks 235 

118. Talus in Rio Grande valley, N.M 236 

119. The formation of residual soil 237 

120. Sand dunes, Cape Ann, Mass . .238 

121. Moqui pueblo, New Mexico . 239 

122. Talus furnishing load to river . 240 

123. Yellowstone valley, broadening by weathering .... 242 

124. Boulder bed of Westfield River, Mass 243 

125. Sea cliffs on volcanic island 244 

126. Granite hill rounded by glacial action 245 

127. Relief map of Eurasia 250 

128. Section across the Atlantic and United States .... 251 

129. Relief map of North America 252 

130. A deep mountain valley 262 

131. Stream issuing from a limestone cave 263 

132. Brink of Niagara Falls 264 

133. Gorge near Ithaca, N.Y 265 

134. Royal Gorge, Col 265 

135. Oxbow cut-off in Connecticut valley 266 

136. Development of the canon 267 

137. Development of the canon profile 267 

138. Development of old valley 267 

139. The Yellowstone, broadening by weathering 268 

140. A bit of Illinois drainage 269 

141. A bit of West Virginia drainage 269 

142. Canon of the Colorado 270 

143. A broad Alpine valley 271 

144. Mountain gorge in the Alps 272 

145. Diagram illustrating change in divide . ... . . . 273 

146. Diagram illustrating change in divide 274 

147. Diagram illustrating monoclinal shifting 274 

148. Diagram illustrating sudden change in divide .... 275 

149. Effect of elevation on Colorado canon 276 

150. The drainage of an arid region 280 

151. The Great Basin 281 

152 Effect of glaciation on stream courses 282 



ILL US TEA TIONS. XXV 

FIG. PAGE 

153. Delta of the Mississippi 286 

154. Mode of formation of a delta . 288 

155. An alluvial fan 288 

156. Floodplain among mountains 289 

157. Floodplain of a great river 290 

158. Meandering of the Mississippi 291 

159. Meandering of the Mississippi 292 

160. Meandering of the Mississippi . 292 

161. Falls of the Yellowstone 293 

162. Taughannock Falls, N.Y. . . 294 

163. American Falls, Niagara . . . 295 

164. Yosemite Falls 296 

165. Falls in a gorge near Ithaca, N.Y. ....... 297 

166. Diagram illustrating origin of Niagara 298 

167. River valley transformed to a lake (Adirondacks) . . . 299 

168. Glacial lakes in the Adirondacks 300 

169. Bird's-eye view of Niagara River 301 

170. Shore lines of extinct Lake Bonneville 302 

171. A Florida swamp 303 

172. Ray Brook, Adirondacks 304 

173. An Alpine snow field 306 

174. "Whitney Glacier, Mount Shasta 307 

175. The Rhone glacier . . . 308 

176. Crevasse in a glacier . . 309 

177. Glacier, Mount Dana, Cal . 310 

178. Section of a glacier 312 

179. Ice cave at terminus of a glacier . . . . . . . 312 

180. Forest at foot of Malaspina Glacier, Alaska 313 

181. A Nunatak in Greenland . 314 

182. Icebergs in the Antarctic 315 

183. An iceberg in water 316 

184. Glacial lakes and moraine in a mountain valley .... 317 

185. Extent of the continental ice sheet in America .... 318 

186. Boulder in moraine, Cape Ann, Mass 320 

187. Bear-den moraine, Cape Ann, Mass 321 

188. Boulder-strewn till soil in Maine 321 

189. Glacial scratches on a pebble 322 

190. Glacial lakes in Massachusetts . 324 

191. Watkins Glen, N.Y 326 

192. Sea cliff, Cape Cod, Mass . .328 

193. Submerged valley on the coast of Mount Desert, Me. . . . 330 



XXY1 



ILLUSTRATIONS. 



FIG. 

194. Ocean bar on the Texas coast 

195. Destruction of Heligoland by the ocean 

196. Lake Spit 

197. Hook, Lake Michigan .... 

198. Sea cave in granite rock, Cape Ann, Mass. 

199. Effect of dike on form of coast, Cape Ann, Mass. 

200. Pond formed by beach barrier, Cape Ann, Mass. 

201. Crescent-shaped beach, Cape Ann, Mass. 

202. Boulders worn from headland by waves 

203. Rocky beach on exposed coast, Cape Ann, Mass. 

204. Mat of seaweed between tides, Cape Ann, Mass. 

205. A mangrove swamp 

206. Salt marsh, Cape Ann, Mass. 

207. Coral reef on the Australian coast 

208. Keys on the Florida coast 

209. An atoll in the Pacific . 

210. Diagram illustrating origin of atolls 

211. The coast of Casco Bay, Me. 

212. Cliff on the shore of Lake Michigan 

213. Lagoon enclosed behind lake beach 

214. Plain in Pecos Valley, N.M. . 

215. Plain in valley of Red River of the North 

216. Taos Mountains, N.M. . 

217. Plateau near Colorado River 

218. Butte in New Mexico . 

219. Talus slope in the Elk Mountains, Col. 

220. Matterhorn, Switzerland 

221. A mountain park (Baker's) . 

222. Mountain gorge in the Peruvian Andes 

223. Mount of the Holy Cross, Col. 

224. Trail on Long's Peak, Col. . 

225. Mountain ridge on the Canadian Pacific 

226. Section across a mountain ridge . 

227. A bit of mountain drainage . 

228. Map of mountain drainage . 

229. Diagram illustrating the development of a mountain 

230. A mountain ridge in Colorado 

231. Vesuvius in eruption, 1872 . 

232. Surface of a recent lava flow 

233. Lake formed by a lava dam . 

234. Volcano in the Lipari Islands 



ILLUSTRATIONS. 



XXV11 



FIG. 

235. Disruption of Krakatoa .... 

236. Vesuvius, from Pompeii .... 

237. Mount Hood — an apparently extinct volcano 

238. Muir's Butte, Cal., — a recent volcano 

239. Fusiyama, a Japanese volcano 

240. Angle of slope of volcanic cones . 

241. Mounts Shasta and Shastina 

242. Mato Tepee, Wyo., — a volcanic neck . 

243. Diagram illustrating the earthquake wave . 

244. Waves of Charleston earthquake . 

245. Earthquake shock in Japan .... 

246. Effect of earthquake in Japan, 1891 

247. Eault line associated with Japanese earthquake of 1891 

248. Crater of Oblong Geyser, Yellowstone Park 

249. Old Faithful Geyser, Yellowstone Park 

250. Etching of hard layer by denudation, Brazil 

251. A cliff in the Yosemite .... 

252. Cliffs in the loess of China . 

253. A wave-worn chasm, Gloucester, Mass. 

254. A rugged granite coast, Cape Ann, Mass. 

255. A sloping granite coast, Cape Ann, Mass. 

256. Effect of hard layers on topography 

257. Signal Butte, Tex 

258. Effect of tilted layers on topography 

259. Form of seacoast in inclined strata 

260. Form of seacoast in inclined strata 

261. Ridge of hard rock, etched by denudation 

262. Topography in region of folded rocks . 

263. A part of the Adirondack forest . 

264. Deforesting of the Adirondacks . 

265. Bare rock exposed by removal of forest 

266. Model of Cumberland Valley, Penn. . 

267. Hachure map . . 



PAGE 

375 

376 
378 
379 
380 
380 
382 
383 
384 
384 
385 
386 
387 
388 
389 
396 
398 
399 
400 
401 
401 
402 
402 
403 
403 
403 
404 
405 
409 
410 
411 
437 
438 



XXV111 



ILLUSTRATIONS. 



PLATES AND CHARTS. 



10. 

11. 

12. 
13. 
14. 
15. 
16. 
17. 
18. 
19. 
20. 
21. 
22. 
23. 
24. 
25. 
26. 
27. 

28. 
29. 



"Watkins Glen, New York 
Isotherms for the year (world) 
Isotherms for the year 1892 (United States) 
Isothermal chart for July (world) . 
Isothermal chart for January (world) . 
Isothermal chart for July (United States) 
Isothermal chart for January (United States) 
Isothermal chart of New York (year) 
Winds and isobars for January (world) 
General circulation of the Atlantic, July 
General circulation of the Atlantic, January 
Rainfall chart of the world 
Rainfall chart of the United States 
Depths of the ocean 
Ocean surface temperature, Atlantic 
Oceanic circulation 

Gulf Stream 

Co-tidal lines 

English Channel tides . 
Earth columns, New Mexico . 
The Bad Lands of South Dakota . 
Relief map of the United States 
Drainage areas of the United States 
Delaware and Chesapeake bays 
Drainage in glaciated region, "Wisconsin 
White Glacier, Alaska ... 
Distribution of volcanoes and ocean 

(world) 

Marble Canon, Colorado River 
Navajo church, Arizona . 



PAGE 

Frontispiece 
facing 50 



54 
55 
56 



facing 



facing 
facing 

57 
58 
59 
70 
72 
73 
facing 117 
119 
161 
181 
facing 183 
188 
Jacing 194 
195 
232 
247 
facina 253 
260 
277 
283 
311 
temperatures 

facing 370 ' 
. 391 
. 397 






ILL US TEA TIONS. XXIX 



ACKNOWLEDGMENT OF ILLUSTRATIONS. 

The following illustrations are from the sources indicated. In some 
cases they have been exactly reproduced, but in others they have been made 
more diagrammatic to suit the needs of this book. Some of the illustrations 
not acknowledged are from photographs or lantern slides, the source of 
which could not be ascertained. 1 

Abbe, U. S. S. S., Annual Eeport for 1890, Fig. 56. 

Agassiz, Three Cruises of the Blake, Plate 15. 

Branner, Journal of Geology, Vol. 1, Fig. 250. 

Brown, C. D. (dealer in photographs, Gloucester, Mass.), Figs. 81, 89, 203, 

204, and 254. 
Buchan, Atmospheric Circulation, Challenger Reports, Plates 2, 4, 5, and 9. 
Calvin, Prof. S., State Geologist of Iowa, Des Moines, — Photograph by the 

Survey, Fig. 131. 
Canadian Geological Survey, Photograph, Fig. 99. 
Challenger Reports, Narrative, Figs. 78, 125, and 182. 
Chamberlin, Third Annual Report, U. S. G. S., Fig. 185. 
Diller, Bulletin 79, U. S. G. S., 2 Figs. 232, 233. 
Dunwoody, Summary of International Meteorological Observations, Figs. 26 

and 48 ; same, Professional Paper IX., U. S. S. S., Plate 13. 
Dutton, Second Annual Report, U. S. G. S., Figs. 137 and 149 ; same, Sixth 

Annual, Plate 29 ; same, Ninth Annual, Fig. 244; same, Monograph II., 

U. S. G. S., Fig. 136. 
Ferrel, Popular Treatise on the Winds, Fig. 34. 
Finley, U. S. S. S., Professional Paper VII., Fig. 54. 
Gannett, Thirteenth Annual Report U. S. G. S., Plate 22. 
Gardner, J. L., 2d, Boston, Mass. (Photographs by), Figs. 98, 116, 8 117, 8 

120, 126, 3 186,3 187, 198, 3 199, 3 200, 201, 202, 3 206, and 255.8 
Gilbert, Monograph I., U. S. G. S., Figs. 151, 154, 155, 170, 197, and 213 ; 

same, Fifth Annual Report U. S. G. S., Figs. 212 and 217 ; same, Annual 

Report Smithsonian Institution, 1890, Figs. 166 and 169 ; same, Geology 

of the Henry Mountains, Fig. 147. 

1 TJ. S. C. S., refers to the United States Coast Survey ; U. S. G. S., to the United States 
Geological Survey; and U. S. S. S., to the United States Signal Service. 

2 Some of these which are referred to the Geological Survey publication were made from 
photographs obtained from the Survey. 

8 Also published by Shaler in Ninth Annual Eeport, U. S. G. S. 



XXX ILL US TEA TIONS. 

Greely, U. S. S. S., Professional Paper II. , Plates 6 and 7. 

Griswold, L. S., Dorchester, Mass. (Photograph by), Eig. 97. 

Guyot, Physical Geography, Fig. 2. 

Hann, Berghaus, Atlas der Meteorologie, Plate 12. 

Hann, Hochstetter, and Pokorny, Allgemeine Erdkunde, Pig. 77. 

Harvard College Astronomical Observatory Engravings, Figs. 5, 11, and 13. 

Harvard College Geological Department, Figs. 215 and 231 (former, photo- 
graph from South Dakota World's Fair Commissioner ; latter, pho- 
tograph by Sommer). 

Haynes, F. Jay, St. Paul, Minn. (Photographer), Figs. 105, 123, 139, 161, 
248, 249. 

Hellmann, Schneekrystalle, Fig. 58. 

Hill, First Annual Eeport, Texas Geological Survey, Fig. 257. 

Hope, J. D., Photographer, Watkins, N.Y., Plate 1 and Fig. 191. 

Jackson Photograph Co., Denver, Col., Figs. 134, 221, 224, 237, 238, and 251. 

James, C. H., Photographer, Philadelphia, Pa., Eig. 108. 

Jukes-Browne, Handbook of Physical Geology, Fig. 195. 

Kent, Great Barrier Reef, Eigs. 79 and 207. 

Kobayashi, Earthquake Observations in Japan, Fig. 245. 

Koppen, Segelhandbuch fur den Atlantischen Ozean (reproduced by Davis, 
American Meteorological Journal, Vol. IX.), Plates 10 and 11. 

Lesley, Coal and its Topography, Figs. 256 and 262. 

Levy and Co., Paris (Dealers in Photographs), Figs. 143, 144, 175, and 220. 

Merriam, North American Fauna, Bulletin No. 3, U. S. Dept. of Agriculture, 
Eig. 68 ; same, National Geographic Magazine, Vol. VI. , 1894, Fig. 72. 

Mills, H. E., Annals, Harvard College Astronomical Observatory, Vol. 31, 
Fig. 53. 

Mills, H. R., Realm of Nature, Plates 16 and 27. 

Mississippi River Commission (Maps), Eigs. 158, 159, and 160. 

Mitchell, U. S. C. S., Annual Report for 1886, Eig. 85. 

Murray and Renard, Challenger Reports — Deep Sea Deposits, Plate 14. 

Nasmyth and Carpenter, The Moon, Fig. 10. 

Newcomb, Popular Astronomy, Eig. 12. 

Newell, Eleventh Census Report on Irrigation, Eigs. 61 and 62. 

Newton & Co., London, England (Dealers in Lantern Slides), Figs. 52, 55, 
71, 106, 181, 205, 209, 234, and 339. 

New York State Weather Bureau, Fifth Annual Report, Plate 8 and Eig. 25 ; 
Figures based on the records of this bureau : 18, 28, 29, 33, 42, and 64. 

Notman (Photographer), Montreal, Canada, Fig. 225. 

Pach (Photographer), New York, N.Y., Eig. 82. 

Peschels (Leipoldt), Physische Erdkunde, Plates 18 and 19. 

Pillsbury, Annual Report, U. S. C. S. for 1890, Plate 17. 



ILL US TEA TIONS. XXxi 

Proctor Bros. (Dealers in Photographs), Gloucester, Mass., Fig. 80. 

Reid, National Geographic Magazine, Vol. IV., Plate 26. 

Richthofen, China, Fig. 252. 

Riggenbach (Photographs), Pigs. 50 and 57 (latter from several sources). 

Ritchie, J., Jr., Boston, Mass. (Photographs by), Pigs. 124 and 188. 

Russell, Pifth Annual Report, U. S. G. S., Pig. 177 ; same, Eighth Annual, 

Fig. 184 ; same, Thirteenth Annual, Figs. 67 and 180. 
Sella (Photographs; Chas. Pollock, Boston, Agent), Figs. 176 and 179. 
Shaler, Twelfth Annual Report, U. S. G. S., Figs. 107, 157, and 171. 
Sigsbee, U. S. C. S., Deep Sea Sounding and Dredging, Figs. 73 and 74. 
Smith, W. M. (Dealer in Photographs, Provincetown, Mass.), Fig. 192. 
Stoddard, S. R. (Photographer), Glens Falls, N.Y., Figs. 88, 167, 168, 172, 

193, 263, 264, and 265. 
Symons, Eruption of Krakatoa, Fig. 235. 
Todd, Bulletin I., South Dakota Geological Survey, Fig. 112. 
Trotter, Lessons in the New Geography, Figs. 127 and 129. 
United States Coast Survey Charts, Figs. 153, 194, 208, 211, 267, and Plate 24. 
United States Geological Survey Photographs, Figs. 66, 94, 95, 96, 119, 122, 

132, 142, 163, 174, 196, 230, 241, 242, 261, and Plate 28 ; same, Topo- 
graphic Maps, Figs. 150, 190, 228, and Plate 25. 
United States Geological Survey of the Territories (Hayden), Photographs, 

Figs. 69, 115, 121, 130, 156, 164, 219, 223. 
United States Hydrographic Bureau (Coast Pilot), Figs. 43, 44, 45. 
United States Signal Service and Weather Bureau, Figs. 30, 31, 32, 46, 47, 

49, 60, 63, and Plate 3. 
Van Bebber, Lehrbuch der Meteorologie, Fig. 41 . 
Walcott, National Geographic Magazine, Vol. V., Fig. 109. 
Ward, Annals Harvard College Astronomical Observatory, Vol. 31, Fig. 51. 
Wild, Thalassa, Fig. 21. 

Willis, Thirteenth Annual Report, U. S. G. S., Figs. 92, 93, and 101. 
Williston, Prof. S. W., Kansas University Geological Department, Lawrence, 

Kansas (Photograph by), Fig. 90 and Plate 21. 



Part I. 

THE A IB. 

WITH AN INTRODUCTORY CHAPTER ON THE ASTRONOMICAL 
RELATIONS OF THE EARTH. 



ELEMENTARY PHYSICAL GEOGBAPHY. 



?>Kc 



CHAPTER I. 



THE EARTH AS A PLANET. 



Form of the Earth. — The earth is a spherical body com- 
posed of three different portions, — a dense central mass, which 
is probably solid, and two envelopes, the ocean and the air. 
The central part has a much greater bulk than either of the 
other portions. In reality the form is not exactly spherical, 
for the diameters of a sphere should have the same length in 
all parts ; but in the earth the diameter at the equator is 26^ 
miles longer than that at the poles, 
where its length is 7899 miles. 
This flattening of the poles gives 
to the earth the form of an oblate 
spheroid instead of a true sphere 

(Fig. 1), 

While this irregularity of the 
earth was detected only after a 
series of very careful measure- 
ments, it is in reality the greatest 
on the surface of the earth; but 
there are other and less extensive 
irregularities, which are much 
more noticeable. These are of two kinds, — continents and 
mountains. The surface rises and falls in a series of great 
wave-like irregularities, which form the continents and ocean 

3 




Fig. 1. 

Diagram showing a section of 
a sphere (heavy line) , and an 
oblate spheroid (dotted line). 



PHYSICAL GEOGRAPHY. 



basins. On the continents, and occasionally in the oceans, 
the surface rises along relatively narrow lines into a series 
of high mountain ridges. Although these are the greatest 
elevations on the earth's surface, and therefore attract our 
attention, they are really very small irregularities when com- 
pared with the continents of which they usually form a small 
portion (Fig. 128). 

Considering the sea level as 0, the highest point on the 
earth is about 29,000 feet in elevation. Depressions of over 
25,000 feet are found in several places in the ocean beds. 
The total range in elevation between the highest mountain, 
and the greatest ocean depth is about 57,000 feet. It can be 
readily seen how small this is in comparison with the earth 
as a whole, when we remember that the diameter of the earth 
at the equator is 41,847,192 feet. Upon a globe of ordinary 
size they could not be shown on true scale. Although there 
are points on the land whose height is greater than the 
deepest known parts of the ocean, the average depth of 
the ocean, which is about 12,000 feet, is much greater than 
the average height of the land, which is approximately 2500 

feet (see Chap. XIV.). 
The greater part of the 
water on the earth's sur- 
face is accumulated in the 
broad hollows between 
the continents. The sur- 
face of this water mass is 
much greater in area than 
that of the land (Fig. 2), 
the proportion being 1 of 
land to 2. 6 of water (roughly 3:8). Late calculations give the 
area of the land as 142,000,000 square kilometers, and of the 
water as 368,000,000 square kilometers. The total volume of 




Fig. 2. 
Land and water hemispheres. 



THE EARTH AS A PLANET. 5 

the water of the oceans is estimated to be 1,347,874,850 cubic 
kilometers. 

There are other smaller irregularities on the surface of the 
earth, and many minor peculiarities, some of which are dis- 
cussed in the later chapters. Surrounding the earth is a 
gaseous envelope, the atmosphere, which extends to an 




Fig. 3. 

The solar system, showing the relative distances from the sun, the direction of 
revolutions, relative size of the orbits, and the number of satellites. 



unknown distance, but which at a height of five or six miles 
from the surface is very much rarified. 

The Solar System. — The earth is one of several bodies 
which together form the solar system. They are a family of 
bodies bound together by the tie of gravitation and engaged 
in a series of movements around a central body, the sun 
(Fig. 3). In the solar system there are five classes of 



6 PHYSICAL GEOGRAPHY. 

bodies. In the center is the sun, the largest of all, and the 
one upon which the others depend more than upon any other 
member. The second class of bodies is that of the planets, 
of which eight are known. These all revolve around the 
sun in orbits which are nearly circular, but not exactly so, 
being in reality, ellipses with the sun at one of the foci. 
The third class of bodies is that of the satellites, of which 
the moon is an example. Most of the planets have satellites, 
which are always much smaller than the planet about which 
they revolve. The earth has but one moon, but some of the 
planets have several. Twenty moons have already been 
discovered, of which all but three belong to the outer group 
of planets, Jupiter, Saturn, Uranus, and Neptune. A fourth 
group of bodies in the solar system is that of the asteroids, 
of which about 400 are now known. These small planets 
revolve about the sun in the space between the orbits 
of Mars and Jupiter. Aside from these members, there is a 
fifth group of irregular bodies, the comets and meteors, 
which move in a manner different from that of the other 
members of the solar system. 

The Sun. — The central and largest member of the solar 
system, the sun itself, unlike the planets, is so constituted 
that it sends out into space a form of energy which produces 
both light and heat. It is the source of much of the energy 
which finds expression upon the surface of the earth in the 
forms of light, heat, and life itself. This immense body is 
fully 92,750,000 miles distant from the earth. 

Since the sun is able to emit rays which produce heat, we 
know that it must be a hot body ; but there is as yet no 
means of telling what its temperature is. Owing to the way 
it affects the movements of the several members of the solar 
system, it is known that the materials composing the sun are 
not so dense as the solid part of the earth. It seems quite 



THE EARTH AS A PLANET. 



certain that at least a large part of the sun is in the form 
of gas. By means of the instrument known as the spectro- 
scope, we have learned much concerning the actual composi- 
tion of the sun. By this instrument it has been found that 
many of the elements known on the earth exist in the sun 
in a gaseous form. 

Since we know very little about the condition of the 
earth on which we live, it is hardly to be expected that our 
knowledge of a body so distant as the sun would be very 
accurate. Still the studies which have been carried on by 
means of the telescope have revealed the fact that there are at 
least three quite different 
parts to the sun. These 
are the corona, which is 
outermost, the chromo- 
sphere, and the photosphere, 
the latter being the densest 
part. It is the portion 
from which the light and 
heat are emitted ; and 
from its surface the diame- 
ter of the sun is about 
860,000 miles (Fig. 4). 
Above the photosphere 
comes the chromosphere, 
which appears to be the 
true atmosphere of the sun. It consists mainly of glowing 
hydrogen gas ; but in its lower portions many metals, such as 
iron, are known to exist in the form of gas. It is in violent 
commotion, as if in eruption ; and the photosphere itself also 
presents signs of violent activity. Extending to a distance 
sometimes as great as 300,000 miles above the surface of the 
sun, is the corona, the character of which is not understood. 




Fig. 4. 

Diagram to show the relative size of the 
sun and the largest planets, 
true scale. 



Drawn on 



8 PHYSICAL GEOGRAPHY. 

Certain peculiar spots known as sun spots (Fig. 5) appear 

upon the surface of the sun and move across its face until 

they disappear on the opposite 

side, being carried around by the 

rotation of the sun. Their origin 

H is not known, but they appear to 

have an influence upon the earth 

'- 1 . ; in at least two ways, one upon 

H atmospheric electricity, the other 

-- fi upon certain climatic features. 

IG ' ' _ The sun is engaged in two mo- 

Sun spots, 1872. ° ° 

tions. It rotates, as do all the 
larger bodies of the solar system ; but the period of rotation 
is not exactly known, though it is somewhere between 25 
and 26^- days. Strangely enough, the period of rotation 
appears to vary according to the latitude. The second mo- 
tion is one in which the entire solar system is engaged ; but 
the amount and exact nature of this is not known. The sys- 
tem is moving through space at an unknown rate, toward 
the constellation Hercules. 

The Planets. — Mercury, the smallest of the planets, is nearest 
to the sun, on the average being about 35,750,000 miles from 
it (Fig. 6). The diameter is a little more than one-third 



Mars Jupiter Saturn Uranus Neptune 

|m-H — — h H — I » • 

•W. V Earth 

Fig. 6. 
Diagram to show the relative distances of the various planets from the sun. 

that of the earth (or 2992 miles), and it rotates on its axis 
in about 24 hours, while it revolves around the sun once 
in about 88 days. We know little concerning the condi- 
tions on this planet. 



THE EARTH AS A PLANET. 




Fig. 7. 
Diagram to show the rela- 
tive size of the smaller 
planets. 



The next body outside of Mercury is Venus, the most 
brilliant of planets. It is almost the same size as the 
earth, being in reality about 250 miles less in diameter 
(7660 miles) (Fig. 7). Some observers 
think that they have detected a rotation 
with a period of a little more than 24 
hours ; but this is doubted by most 
astronomers. The period of revolution 
is considerably less than ours, or about 
225 days. It appears quite certain that 
there is an atmosphere upon this planet, 
and so far as we can tell, it closely 
resembles ours. No satellite is known 
to exist. 

Outside of the earth, which is the 
next planet in the solar system, comes Mars, which next 
to Mercury, is the smallest of the planets, having a diameter 
of but little more than 4200 miles. Its time of rotation is 
a little over 24|- hours, and its revolution about the sun 
is accomplished in nearly 687 days. Its mean distance from 
the sun is 141,000,000 miles. The axis of Mars is inclined 
about 27° to the plane of its orbit, which is about 4° more 
than the inclination of the earth's axis. There are two tiny 
satellites, one less than 10 miles in diameter, the other 
perhaps twice that size ; and the latter is not more than 
4000 miles from the surface of the planet, about which 
it revolves in a period of 7 h. 39 m. 

Jupiter, the largest of planets (Fig. 4), has a mass greater 
than that of all the others combined, the mean diameter being 
about 86,000 miles ; but the diameter at the equator is fully 
5000 miles greater than that at the poles. The volume 
of Jupiter is about 1300 times that of the earth. On the 
average, the distance from the sun is about 480,000,000 



10 PHYSICAL GEOGRAPHY. 

miles, and it takes nearly 12 years for it to make a revolu- 
tion about the sun. The time of rotation is a very little 
over 9 h. 55 m. 

It is evident that what we see with the telescope is not 
the surface of the planet, but a dense atmosphere of some 
form of cloud. Therefore we have no means of knowing 
what the actual condition of Jupiter is, though we may infer 
that the planet is still heated, and that the clouds which 
we see are the result of this heated condition. Five moons 
revolve about Jupiter, the most distant being 1,162,000 miles 
from the planet, while the nearest is only a little farther 
away than our moon is from us. 

Next beyond Jupiter is Saturn, the second largest of the 
solar planets. Its distance is 881,000,000 miles from the 
sun, around which it revolves in about 29|- years, while it 
rotates upon its axis in 10 h. 14 m. 1 This planet has eight 
moons ; but the most remarkable feature connected with it, 
is its surrounding group of flattened rings, whose inner 
diameter is 100,000 miles. The telescope has not yet defi- 
nitely revealed the nature of these rings. 

As the distance from the earth increases, our knowledge 
of the members of the solar system becomes less accu- 
rate. Hence, since its mean distance from the sun is fully 
1,771,000,000 miles, Uranus is scarcely known. It revolves 
about the sun once in 84 years, but its period of rotation 
is not known. There are four satellites. 

Until 1846 no other large planet was known; but as a 
result of prediction, Neptune was discovered in that year. 
The discovery of this planet is one of the most remarkable 
proofs of the accuracy of the theory of gravitation; for it 

1 It will be noticed that as the distance from the sun increases, the time 
required for a revolution also increases, while the period of rotation rapidly 
decreases. 



THE EABTH AS A PLANET. 11 

was determined by irregularities in the movement of Uranus, 
that another planet must exist outside of its orbit; and after 
•careful calculations, the place where this planet could be 
found was predicted, and Neptune was discovered at a mean 
distance of 2,775,000,000 miles from the sun. One moon 
has been detected. 

Asteroids. — In the year 1801, a small planet known as 
Ceres was discovered in the space between Mars and Jupiter. 
Since that time about 400 other smaller bodies have been 
found in the same general region. In no cases have these 
small planets a diameter greater than 520 miles, while the 
smallest that have been discovered have diameters of less 
than 40 miles. Their movement through space is some- 
what irregular; and there have been many speculations con- 
cerning their origin, though as yet no satisfactory explana- 
tion has been advanced. 

The Earth. — While cold at the surface, we have many 
reasons for believing that the interior of the earth is highly 
heated. Proof of this is found in the facts that at the 
surface, volcanoes emit quantities of molten rock which come 
from below, and that in all deep mines and well-borings the 
temperature of the rocks is found to increase at a moderately 
uniform rate, on the average 1° for about every 50 or 60 
feet of descent. If this rate of increase continues, the rocks 
at a depth of less than 100 miles are so hot that they would 
be molten under the conditions which exist at the surface. 

It was once believed that the interior of the earth was in 
a molten condition, and that the solid surface was merely 
a crust resting upon this liquid sphere; but many facts now 
lead us to the belief that the interior is at least as rigid as 
steel. The proof of this has been furnished by the studies of 
physicists and astronomers. At present we are forced to the 
belief, that although highly heated, the rocks in the interior 



12 



PHYSICAL GEOGRAPHY. 



of the earth are prevented from melting by the great pres- 
sure of the overlying layers; and by this theory we are able 
to satisfactorily account for all of the phenomena that 




Fig. 8. 
Diagram illustrating the cause of seasons. 

formerly seemed to demand the explanation of a liquid 
interior. 

The earth is engaged in a number of movements in 
space. It revolves around the sun in about 365.24 days, in an 



THE EARTH AS A PLANET. 13 

orbit which is nearly a circle; but instead of being actually 
a circle with the sun at its center, the orbit is really an 
ellipse with the sun at one of the foci. Therefore, in the 
course of its revolution, the earth is at one time farther from 
the sun than in the opposite season, the distance now vary- 
ing between 91,000,000 and 94,000,000 miles, with an average 
distance of about 92,750,000 miles. 

During the revolution, the earth rotates about one of its 
diameters, which we call the axis, and this rotation occu- 
pies a little less than 24 hours (23 h. 56 m.), or one 
day. This rotation causes the familiar alternation of day 
and night; and if the earth's axis were at right angles to the 
plane of revolution, the day and night would be equal in 
length; but since it is inclined to this plane at an angle of 
23° 27', the relative length of day and night varies from 
day to day. Indeed, the seasons themselves depend upon 
this inclination of the poles (Fig. 8); for in the course 
of a revolution, the pole is always pointed toward a certain 
part of the heavens ; and as the earth moves about the sun, 
the northern hemisphere alternately faces and is turned away 
from the sun. When turned toward the sun, the summer 
season is caused, and when turned away from it, the winter 
season results, because the solar rays then fall less vertically 
upon the hemisphere, and the length of the day is shorter. 
Between these two opposite seasons we have spring and 
autumn. 

The Moon. — This, the nearest to our earth of all the 
heavenly bodies, has an average distance of about 240,000 
miles, and a diameter of 2160 miles (Fig. 9). Since the path 
of the moon about the earth is an ellipse with the earth at 
one of the foci, the distance varies ; but it is rarely more 
than 253,000 miles nor less than 227,000 miles distant. 
When farthest from the earth it is said to be in Apogee, 



14 



PHYSICAL GEOGRAPHY. 




Fig, 9. 

The relative size of 
earth and moon. 



and when nearest in Perigee ; and once in every revolution 
Apogee and Perigee are reached. 

Aside from those it makes in company with the earth, its 
two important movements in space are a revolution around 
the earth and a rotation about an axis, both 
of these movements occurring in the same 
period of time, or 29^ days. Therefore 
one side of the moon is never seen from the 
earth. Also, as a result of this condition, 
the length of the lunar day is 29J of our 
days ; and therefore at the lunar equator 
the sun shines steadily for nearly 15 days 
and is absent an equal length of time. 
Under these conditions the surface of the moon is warmed 
during the long day, and at night becomes cooled down to 
temperatures which 
are perhaps as low as 
- 200°. 

There is no atmos- 
phere and no ocean 
on the moon ; and 
the only change upon 
the surface seems to 
be that between con- 
ditions of heat and 
cold, and light and 
darkness. It emits 
an almost impercepti- Fig- 10 - 

ble amount of radiant Lunar craters, the largest being Gassendi. 

energy, and the light from the moon is reflected sunlight. 1 
As a result of the careful telescopic study of the moon, 

1 Direct light from the sun is 600,000 times as strong as that which is 
reflected from the moon. 




THE EARTH AS A PLANET. 15 

astronomers have been able to map many of the details of lunar 
topography, with considerable accuracy, and even to measure 
mountain heights. While there are other striking topo- 
graphic features, the most notable thing about the lunar land- 
scape is the great number of crater-like mountains, which bear 
a certain resemblance to the volcanoes on the earth's surface, 
excepting that many of them are of immense size (Fig. 10). 
Comets, Shooting Stars and Meteors. — Aside from those 
described, which may be considered the normal members of 
the solar system, there are other heavenly bodies which 
do not appear to be regular parts of the system. The 
strangest of these are comets. Some 500 of these have 
been recorded as visible to the naked eye ; and in addition, 
over 200 have been detected by the aid of the telescope, some 
of these being millions of miles in length. When near the 
sun, they usually have a relatively dense head and a vaporous 
tail, through which stars are visible (Fig. 11). Some have 
regular elliptical orbits, and 
their time of appearance can 
be closely calculated; but 
the orbits of others are ap- 
parently parabolas, so that 
if they ever return to the 
solar system, it is only after 
long periods of time have 
elapsed, and after having 
made a journey far beyond ^ ^^ ^ 

the outermost limits of the 

solar system. Perhaps these may be mere wanderers through 
space, which after one visit to the solar system, depart never 
to return again. What they are, whence they came, whither 
they are going, or what relation they bear to the solar sys- 
tem, is still an unsolved mystery. 




16 PHYSICAL GEOGRAPHY. 

Comets have an added interest to us, from the fact that 
some shooting stars and meteors seem to be remnants of 
comets, which at some former time have crossed the orbit 
of the earth. Thus the November meteorites are due to the 
fact that in its movement around the sun the earth en- 
counters particles that are left in the trail of a comet (Tem- 
pers) which has a period of revolution of about thirty-three 
years ; and the August meteors (Fig. 12) appear to have 
a similar origin. 

Meteors and shooting stars (meteors are large shooting 
stars) enter the earth's atmosphere at a high rate of speed, 

and are burned up 
■* •, 0/ August Mehpre in the higher lay ers 

of the atmosphere, 
often at an eleva- 
tion as great as 100 
miles from the sur- 
face of the earth. 
This burning is 
the result of fric- 
tion with the air, which produces a high heat, because in 
addition to the movement of the meteor, there is often 
added the motion of the earth itself, which is about 98,000 
feet a second. Hence in small bodies, the burning is almost 
instantaneous ; but some of the larger meteors pass entirely 
through the atmosphere, and reach the earth's surface. 

A study of these rather rare meteorites, reveals to us the 
very interesting fact that no new element exists in them ; 
and therefore we may fairly conclude that the elements 
composing comets are the same as some of those which make 
up the earth's crust. In watching the heavens at night, 
scarcely an hour can pass without noticing shooting stars ; 
and since the same would probably be true of the day if we 




Fig. 12. 
Orbit of the second comet of 1862. 



THE EARTH AS A PLANET. 



17 



could then see them, we conclude that there are immense 
numbers of these bodies in the space through which the earth 
travels. 

The Stellar System. — Far away in space, many times 
farther than the sun is from us, innumerable stars are 
scattered. Already many thousands are known, and it is 
estimated that over 30,000,000 are visible with the telescope. 
Like the sun, they emit an energy which produces both 
light and heat ; and it is very probable that many, if 
not all, have planetary bodies revolving about them. 
One satellite, that belonging to Sirius, has already been dis- 
covered ; and some double 
stars are known to be re- 
volving about a common 
center of gravity. The 
distance between the stars, 
and even between the earth 
and the nearest stars, is im- 
mense, and in most cases in- 
calculable. If each star is 
a sun with accompanying 
planets, and if each of these 
suns is as far from its near- 
est stellar neighbors as we 
are from ours, the immensity 
and grandeur of the system 
transcends our imagination. 

The stars are arranged in 
a disc-like belt, the greatest 
diameter of which is in the direction of the Milky Way. 
At right angles to this there is a zone of abundant nebulce, 
(Fig. 13), although these strange bodies are not absent from 
other parts of the heavens. Some have conjectured that 




Fig. 13. 
Andromeda nebula, from a drawing. 



18 PHYSICAL GEOGRAPHY. 

nebulae are other stellar systems, so distant from us that 
the individual members cannot be separated by our tele- 
scopes ; but the spectroscope seems to show that they are 
bodies of glowing gas, and this has an important bearing 
upon the nebular hypothesis, which we soon discuss. One 
very important thing concerning both stars and nebulas, is 
that the spectroscope has detected in them many of the 
elements which we find upon the earth. 

A question of very deep interest, is whether the stars form 
a great system in which the individual members are inter- 
related, as is the case among the members of the solar 
system ? Unfortunately, in the present state of science, we 
are unable to return a definite answer to this question. 

Symmetry of the Solar System. — In theorizing upon a 
basis of known facts we must confine ourselves to the solar 
system ; and it is interesting to note the wonderful symmetry 
of arrangement and the beautiful order which exists here. 
Throughout the entire system, the law of gravitation prevails 
and governs the movements of all the bodies, each member 
attracting the other in direct proportion to the product of 
the masses and inversely proportional to the square of the 
distance. The regular members of the system are all nearly 
spherical, and they rotate about an axis and revolve in an 
orbit which is nearly circular. In direction of rotation and 
revolution there is a marked uniformity, as there is also in 
the plane of revolution. 

All of these regularities of behavior, take place notwith- 
standing the fact that immense distances separate the various 
bodies, and that this space is practically void. We can form 
no accurate conception of these immense distances ; but the 
following quotation from Newcomb's Astronomy furnishes 
some idea of this : — 

"To give an idea of the relative distances, suppose a 



THE EARTH AS A PLANET. 19 

voyager through the celestial spaces could travel from the 
sun to the outermost planet of our system in twenty-four 
hours. So enormous would be his velocity, that it would 
carry him across the Atlantic Ocean, from New York to 
Liverpool, in less than a tenth of a second of the clock. 
Starting from the sun with this velocity, he would cross the 
orbits of the inner planets in rapid succession, and the outer 
ones more slowly, until, at the end of a single day, he would 
reach the confines of our system, crossing the orbit of Nep- 
tune. But, though he passed eight planets the first day, he 
would pass none the next, for he would have to journey 
eighteen or twenty years, without diminution of speed, 
before he would reach the nearest star, and would then have 
to continue his journey as far again before he could reach 
another. All the planets of our system would have vanished 
in the distance, in the course of the first three days, and the 
sun would be but an insignificant star in the firmament." 

The sun in the center of the solar system is a true star, in 
many respects like the others which dot the firmament. 
This being the case, may we not fairly speculate as to the 
possibility of other worlds and systems like our own, far 
away in space, even to the outermost limits which can be 
reached by the human vision ; and if this be so, how vast is 
the universe, and how insignificant the small cold body of 
matter upon which we dwell ! 

The Nebular Hypothesis. — Before many facts concerning 
the universe were known, the philosopher Kant proposed a 
hypothesis to account for the origin of the solar system ; and 
later, Herschel and Laplace proposed an explanation which 
in many respects was like that of Kant. We know this 
explanation under the name of the nebular hypothesis. 

By this it is assumed that the space occupied by the 
members of the solar system, and probably even to a con- 



20 PHYSICAL GEOGRAPHY. 

siderable distance beyond this, was occupied by a nebulous 
mass of highly heated vapor. It is one of the laws of nature 
that radiant energy passes from warmer to colder bodies, and 
that by this radiation a contraction and condensation neces- 
sarily follow. This nebulous mass, composed of all the ele- 
ments which now enter into the composition of the various 
members of the solar system, during the process of cooling 
separated into rings which were the parents of the several 
planets. As the mass lost heat and began to condense and 
contract, the materials began to accumulate about some 
denser part of these rings, the accumulations about these 
denser portions being determined by the fact that gravita- 
tive action was stronger there than elsewhere. 

As a result of this accumulation about centers, the original 
nebulous mass became broken up into several smaller masses 
of similar nature ; and by a continuation of the process other 
rings were thrown off, out of which the satellites were 
formed. Original motion about a central portion of the 
nebula has naturally been inherited and is now indicated 
by the movements of the bodies in the solar system. The 
cooling of these bodies is still in progress, and different 
members of the system have reached different stages. 

Verification of the Nebular Hypothesis. — While we cannot 
state that this theory is definitely proven, many facts point 
to its truth as a general explanation of the solar universe. 
For instance, it would account for the fact that the planets 
move about the sun in a common direction, and that the 
planes of revolution are nearly the same in the different 
planets (the inclination in no case being more than a few 
degrees) . This similarity also extends even to the satellites ; 
and the rotation of the bodies whose rotation has been 
determined has the same kind of uniformity. All of the 
orbits of the members of the solar system are ellipses 



THE EARTH AS A PLANET. 21 

approaching a circle. This together with the uniform action 
of gravitation suggests a common origin. 

The fact that all the bodies regularly belonging to the 
solar system are nearly spherical in form is suggestive ; and 
this form can readily be accounted for if the bodies were 
once liquid. A former liquid condition is suggested by the 
fact that those bodies which are well known, all have a 
larger diameter at the equator than at the poles, although 
it is true that this may be explained in other ways. Then 
also, signs of heat are plainly seen in some of the mem- 
bers of the solar system ; and in the smaller bodies these 
signs are less apparent. Thus the sun is highly heated; 
Jupiter, Saturn, and other of the outer planets show signs 
of considerable heat ; the earth is cold at the surface, and 
hot in the center ; Mars, Venus, and Mercury are cold at the 
surface ; and the moon appears to be entirely cold. 

Upon the nebular hypothesis, we should expect that the 
density of the members of the solar system would increase 
from the outer bodies toward the center ; and this actually 
is the case, the only exceptions being the easily explained 
cases of Saturn and the sun. There are other reasons for 
believing in the nebular hypothesis. So far as we may 
judge from the results of spectroscopic study and from 
the examinations of meteorites that have fallen upon the 
earth, the bodies in the solar system are composed of the 
same elements as those which make the earth ; and this sug- 
gests that they have been made from the same original mass. 

Far away in space, beyond the solar system, we even find 
nebulous masses of gas which are exactly like those out of 
which the solar system is believed to have been made ; and 
in some of these nebulae the condensation into planetary 
bodies appears to be in progress (Fig. 13). Nearly every 
gradation has been found between this kind of nebula and 



22 PHYSICAL GEOGRAPHY. 

that which is apparently one mass of glowing gas. It is 
not improbable that even now other worlds are in process 
of formation in the far distant regions of space. 



REFERENCE BOOKS. 1 

Newcomb. — Popular Astronomy (school edition). Harper Brothers, New 
York. Seventh edition, 1894. 8vo. Published also in larger form. 
School edition, $1.30 ; larger book, $2.50. (General and quite elementary.) 

Lockyer. — Elementary Lessons in Astronomy. Macmillan & Co., New 
York. 8vo. $1.25. (General and elementary.) 

Chambers. — Handbook of Descriptive and Practical Astronomy. Mac- 
millan & Co., New York. Fourth edition, 1889. 8vo. Three volumes. 
Vol. I., $5.25 ; Vol. II., $5.25; Vol. III., $3.50. (Large and comprehen- 
sive.) 

Proctor and Ranyard. — Old and New Astronomy. Longmans, Green, & 
Co., New York, 1892. 8vo. $12.00. (Complete and well illustrated.) 

Young. — The Sun. International Scientific Series. Appleton & Co., New 
York, 1893. 12mo. $2.00. 

Lockyer. — The Chemistry of the Sun. Macmillan & Co., New York, 1887. 
8vo. $4.50. 

Nasmyth and Carpenter. — The Moon. Murray, London (Scribner, New 
York agents), 1885. 8vo. $8.40. (Many remarkable photographs.) 

Neison. — The Moon. Longmans, Green, & Co., New York, 1876. 8vo. 
$10.00. (Well illustrated.) 

Lockyer. — The Meteoritic Hypothesis. Macmillan & Co., New York, 
1890. 8vo. $5.25. (Suggestion of modification of the nebular hypothesis.) 

Scheiner (translated by Frost). — A Treatise on Astronomical Spec- 
troscopy. Ginn & Co., Boston, 1894. 8vo. $5.00. 

1 In giving the publisher's name, the real publishing house is often not mentioned. 
Wherever possible American houses are given, and since some of these act as agents for 
European houses, the name of the agent will at times appear in the place of the English 
publisher. 



CHAPTER II. 



THE ATMOSPHERE. 



General Statement. — Outside of the solid earth, and ex- 
tending to a distance of several hundred miles above it, is a 
gaseous envelope, which we 
know as the atmosphere (Fig. 
14). Its density decreases from 
the surface of the earth toward 
the upper portions; and at a 
height of five miles it is very 
much rarefied. That it ex- 
tends to this great height is 
shown by the fact that meteors 
become white hot by friction 
with it, even at a greater dis- 
tance than this from the earth. 
Fully one-half of the mass of 
the atmosphere is within four 
miles of the surface of the 
earth ; and two-thirds of it is within six miles of the surface 
(Fig. 15). 

The atmosphere is composed almost entirely of two gases, 
nitrogen and oxygen, in the proportion of about 79 to 21. 
These gases are not in chemical combination, but are 
mechanically mixed. Nitrogen is a very inert element, while 
oxygen is active in the production of many changes, and from 

23 




The earth with its atmospheric envel- 
ope, drawn to scale. 



24 PHYSICAL GEOGRAPHY. 

this standpoint the nitrogen of the air may be considered as 
an adulterant of the active oxygen. In additf.on to these 
gases there is a comparatively small amount (about 0.03 per 
cent) of carbonic acid gas, the percentage varying some- 
what according to the location. Its percentage increases in 
the vicinity of volcanoes and large cities. 1 

Beside these three gases there are minor and variable quan- 
tities of other substances ; but of these, only two, water vapor 
and dust particles, are of sufficient general importance for 
consideration here. The term "dust" includes a great 






C^-icS'w^'S ^y^*5Sc/^-;::V"c •■"! 



Fig. 15. 
Diagram to illustrate decrease in density of the atmosphere. 



:;:;,:;, 



variety of substances, such, for instance, as microbes, smoke 
particles, and true dust, which is borne into the air by the 
winds. It seems certain that dust is of much importance in 
the formation of rain and fog. 

Water is readily evaporated, and hence at all times there is 
some water vapor in the air; but the amount depends upon a 
variety of circumstances, chiefly the temperature of the air 
and the presence or absence of bodies of water. With a 

1 While this book is in preparation, the discovery of a new constituent of 
the atmosphere is announced. This, which is called argon, may be a new 
element, but it is now too early to state anything definite about this sub- 
stance. 



THE ATMOSPHEBE. 26 

given amount of moisture, the higher the temperature, the 
greater the rate of evaporation ; but even at temperatures 
below freezing-point small quantities of water vapor may be 
present. 

The atmosphere is of great importance in many respects. 
It distributes the light which comes to us from the sun. It is 
set in motion by the solar energy, and by this means distrib- 
utes heat over the earth. As a result of the effect of solar 
heat upon the atmosphere a great variety of phenomena, such 
as winds, storms, clouds, etc., are produced. These cause 
many changes upon the surface of the earth, and directly and 
indirectly the air makes the earth a place fit for habitation. 

Light. — We obtain light from several sources, — the sun, 
the stars, and the moon and planets. Light from the latter 
source is merely reflected sunlight, and it is small in amount. 
That which comes from the stars is radiated from them 
directly, but it also is insignificant in comparison with that 
received from the sun. 

Solar light, when it reaches the lower layers of the atmos- 
phere, produces the impression upon the eye which we know 
as white; but it has been shown that it probably has a bluish 
tinge before its passage through the air. According to the 
undulatory theory, light passes through the space between 
us and the sun at a very rapid rate in the form of a series of 
waves of ether. It is made up of many waves of different 
lengths, the combination of which gives white. When 
separated, these appear as different colors, and in the rain- 
bow we recognize seven primary colors with intermediate 
hues. The violets and blues have the shortest vibrations, 
and the yellows and reds the longest. As a result of the 
effect of the atmosphere upon these parts of white light 
many optical phenomena are produced. 

If there were no atmosphere, the earth's surface would be 



26 



PHYSICAL GEOGRAPHY. 



illuminated only where the direct rays of the sun fell. The 
atmosphere serves to diffuse light and to render the darkness 
of shadows less intense. This diffusion of light in large 
measure depends upon the amount of solid or liquid impuri- 
ties in the air. In its passage through the air, certain of the 
rays are diffused more readily than others by the process of 
selective scattering. It is those rays that have the shortest 
wave lengths that are thus scattered ; and hence it is that 
the sky is ordinarily blue. The intensity of the blue is great- 
est when coarse dust impurities are least abundant, as is the 
case when the air is clear and dry. If dust particles happen 
to be very abundant, even the coarser rays of yellow light 
may be scattered ; and under rare conditions of very smoky 
air the entire sky may assume a brassy color. Since the 
light is obliged to travel through a greater distance of ais 
near the time of sunset than in midday, the color of the 
western sky in the late afternoon is often yellow, while that 

of midday was a 
dull hazy blue (Fig. 
16). 

Among the most 
beautiful of light 
effects in the atmos- 
phere is that of the 
sunset colors, which 
are due to the scat- 
FlG - 16 - tering of the waves 

Diagram to show that the sun's rays pass through a „>, ; n y. hc^ra + >> a 

,i .. j. , 1 , t WiliOll lldvc bile 

greacer thickness of atmosphere at sunset and sun- 
rise than at midday. (Thickness of atmosphere smaller lengths. As 
greatly exaggerated). a ^^ Qf ^ ^ 

coarser yellows and reds come to us, the reason for the scat- 
tering being the fact that the light at the time of sunset and 
sunrise passes through a great thickness of air, and hence the 




THE ATMOSPHERE. 27 

waves encounter a greater number of dust particles. When 
the atmosphere contains much dust, the morning and evening 
colors are often very intense, but an increase in the quantity 
of dust beyond a certain point tends to dull the tints. With 
clouds in the horizon at sunset or sunrise, these colors of red 
and yellow are often reflected in infinite variety of shade 
and tint. Other phenomena, such as the twilight arch, the 
glow and the afterglow, are associated with the setting of 
the sun. 

Another property of light is that of reflection, and as a 
result of this many interesting optical effects are produced. 
The light of the moon depends upon the reflection of sun- 
light from its surface. The earth also reflects light, and 
this is one of the reasons for the illumination of places that 
are in the shadow of the direct rays of the sun. Other 
places which are illuminated reflect some of their light to 
the parts that are in shadow. Clouds also reflect the light 
of the sun ; and on summer days, when great banks of 
clouds rise high in the air, their surfaces are brilliantly 
illuminated and beautiful cloud effects are produced. 

Another effect of reflection is the mirage, which occurs *f- 
when the air near the surface is warmer than the layers 
above it, and when the reflection from this warm air layer 
reaches the eye of the observer. It often gives rise to an 
appearance like that of a sheet of water ; and travelers in 
desert lands, where this phenomenon is common, are often 
led to think that they are actually approaching a lake. One 
very commonly sees such an appearance as this at the sea or 
lake shore when distant coasts appear to rise above the sur- 
face of the water. It sometimes happens that light is 
reflected from a warm layer which is above the observer ; 
and then the objects appear upside down. This " looming," 
as it is called, is particularly common in Arctic regions ; and 



28 PHYSICAL GEOGRAPHY, 

the effect produced is so fantastic and wonderful that nearly 
all Arctic explorers describe it. 

The rainbow is a phenomenon which partly depends upon 
the reflection of sunlight ; but it is chiefly due to refraction. 
the result being a separation of the several components of 
white light into the colors of the spectrum. Each person 
sees a different rainbow even though two observers may 
stand side by side. The cause for the phenomenon is the 
effect of raindrops which, being denser than the air, bend 
and separate the rays of white light so that we see the 
component colored rays, just as we do when a sunbeam passes 
through a prism. A rainbow is often produced in the spray 
that rises at the base of a waterfall, and at the distance of 
only a few yards one may see it outlined in the spray. 

Another phenomenon resulting from the combined action 
of refraction and reflection is the ring of light or halo which 
often surrounds the sun or moon when their light passes 
through thin hazy clouds in the upper atmosphere. These 
clouds are composed of ice particles, which act upon the light 
in a manner analogous to the effect of raindrops in the 
production of the rainbow. Very remarkable halos are 
formed, particularly in Arctic regions, where the air is often 
filled with minute crystals of ice. Sometimes rings of light 
of very brilliant colors are thus produced. The interference 
with light resulting from the presence of water or ice in 
clouds often produces a ring of light immediately around 
the sun or moon. These are called coronas, and they are 
often beautifully colored, the colors being arranged in con 
centric rings with the red on the outside. 

One of the most important of the phenomena of light is 
that of absorption. Many bodies, such as pure air and water, 
allow most of the rays of light to pass through them with little 
change, and such bodies are called transparent. Other sub- 



THE ATMOSPHERE. 29 

stances are only partially transparent, and we know them 
under the name of translucent bodies. Still others which we 
know as opaque do not allow any light to pass. Thus objects 
have a red color when they reflect a greater number of the 
red than of the other rays ; and other colors are produced in 
the same way by the absorption of different proportions of 
the rays. 

Electricity and Magnetism. — There are certain phenomena 
of magnetism in the earth which some believe to exercise a 
decided influence upon the atmosphere. The earth is a great 
magnet, and the region of greatest magnetic attraction is 
near Hudson's Bay, toward which the needle of the compass 
points in our hemisphere. This may be called the north 
magnetic pole. The magnetic condition of the earth is con- 
stantly changing, both in small daily variations and in 
annual changes, as well as in variations covering many 
years. Occasionally there are magnetic storms, when there 
is a disturbance of magnetic instruments, and when the 
aurora sometimes develops in wonderful complexity and 
weird beauty. This is some electrical effect in the thin 
apper atmosphere ; but our knowledge of this phenomenon 
is obscure. 

Electricity is produced in the atmosphere by various 
causes, and it is nearly always present ; but only rarely does 
it develop sufficient intensity to become visible to the eye. 
In thunderstorms and tornadoes, when the air is in violent 
commotion, there is often sufficient electricity to cause vivid 
discharges from one cloud to another, or to the earth. This 
lightning is an interesting phenomenon, but it does not appear 
to have an important influence in the formation of the storms, 
being really a result of them. The accompanying sound 
is often changed to a rumble by reverberation and echoes 
among the clouds, and between them and the earth. Often 



30 PHYSICAL GEOGRAPHY. 

in violent thunderstorms the air is filled with a constant 
roar of thunder. The lightning spark or bolt is sometimes 
a single large spark, or it may divide and sub-divide, giving 
a branching type of discharge ; and many interesting irregu- 
larities of direction, color, and form are produced. 

The light from the flash moves with great velocity while 
the sound of the thunder travels slowly, at the rate of 
ordinary sound waves. The sound wave is readily worn 
out, and at a distance of a few miles lightning produces 
no perceptible sound. Seat lightning is often the result 
of the reflection among the clouds, or on the horizon, of 
lightning in some far-distant thunderstorm, perhaps en- 
tirely hidden behind the curvature of the earth. 

Heat. 1 — Aside from the heat which comes to us from the 
sun, we obtain a certain small but more constant supply from 
the other bodies of space and from the earth itself; but 
these are relatively unimportant. The radiant energy from 
the sun travels at an enormous velocity as a series of waves, 
which are radiated out from the sun in all directions ; and 
only that small portion of them is received by the earth 
which it happens to intercept in its passage about the sun. 

Some substances allow this energy to pass through them 
with readiness, and these are said to be diathermanous ; 
others absorb it ; and still others reflect the greater part 
of the rays that come to them. The air is comparatively 
diathermanous, as indeed most transparent substances are. 
The smooth glassy surface of water is a good illustration 
of a substance that reflects much of the radiant energy 
coming to it. On the other hand, while the earth reflects 
some, it absorbs a large quantity of heat ; and this is 

1 The sun is emitting a form of energy which under favorable conditions be- 
comes heat, while under other conditions it takes the form of chemical energy 
These rays are therefore properly radiant energy until transformed to heat. 



THE ATMOSPHERE. 31 

particularly true for parts of the earth which are dark 
in color. 

The rays that enter the atmosphere pass through it with 
little interference, because it is diathermanous ; but if there 
is much dust or water vapor in it, a considerable share of the 
rays are intercepted. Thus clouds effectually check the 
passage of many of the rays, and hence cloudy summer days 
are cool. The same effect is produced by a very hazy atmos- 
phere, and in the late afternoon when the solar rays pass 
through a great thickness of air (Fig. 16), the amount of 
heat that reaches the earth is very much less than that 
which comes to the surface at midday. 

Since different parts of the earth's surface behave dif- 
ferently toward the radiant energy, there is much varia- 
tion in the effect produced. This is particularly well 
illustrated by the very marked difference in behavior be- 
tween water and land. The rays that reach the water sur- 
face are in part reflected back into space and thus lost, so 
far as the earth is concerned. Much of that which remains 
raises the temperature of the water ; but as the specific 
heat of the water is high, its temperature is raised very 
slowly. Some is used in the evaporation of the surface 
layers ; and in that case the solar rays are transformed to 
the so-called " latent heat," 1 which does not become appar- 
ent until the vapor is condensed to water. Moreover, the 
water surface is in motion ; and this tends to distribute the 
heat, and thus to prevent the excessive warming of the ocean 
surface. Therefore for these various reasons, even at the 
equator the ocean surface remains relatively cool. 

On the other hand, land reflects very little of the radiant 
energy, and it is a solid body, in which neither evaporation 

1 The old term is still used, though perhaps heat of vaporization would 
be better. 



32 PHYSICAL GEOGRAPHY. 

nor motion is possible. The earth is distinctly not diather- 
manous, and the greater part of the rays which reach it are 
absorbed by the surface portions. Therefore during the 
day the ground tends to become warmed by absorption ; 
and this peculiarity is responsible for many of the phenomena 
of the atmosphere, which are later described. 

Pure air is very slightly warmed by the passage of the 
direct rays of the sun. The small amount of heat thus 
obtained is slightly increased by a supply received from the 
rays which the earth reflects ; but much more is obtained 
from the supply which the earth absorbs. All bodies in 
space are radiating a form of energy, either that which 
belongs to them or that which is radiated to them ; there- 
fore the earth is at all times emitting rays by direct radi- 
ation. During the daytime the amount radiated is less 
in quantity than that received from the sun ; but at night, 
when this supply is cut off, the process of radiation proceeds 
so far that the earth loses much of the heat which it had 
received. Radiation is interfered with by the presence 
of clouds or dust; and hence nights which are cloudy or 
hazy are warmer than those which are clear. 

By the process of conduction, all bodies which are warmed 
tend to transmit their energy to cooler portions. This is 
well illustrated when a cold iron is placed upon a warm 
stove. In the same way, the air in contact with the warmer 
earth is thus warmed by conduction ; but neither air nor 
earth are good conductors of heat, and if this process were 
unaided, the effect would be slight and confined to those 
lower layers of the air which were almost immediately in 
contact with the earth. It is a property of gases that when 
heated they are expanded and thus made lighter. By this 
means a process of convection is started which bears some 
analogy to the boiling of water, and the warm lower layers 



THE ATMOSPHERE. 33 

of air rise above the surface, because the colder and denser 
air forces the lighter layers to ascend. 

The process of convection is one of the most important 
in meteorology ; for upon it in large measure depends the 
development of the winds and other features of atmospheric 
circulation. When air rises it expands, and in the process 
of expansion necessarily cools, the rate of cooling being 
1.6° for every 300 feet of ascent; and descending air, as 
a result of compression, becomes warmed. This feature of 
cooling on ascension gives rise to the formation of many 
of the clouds and rainstorms. 

Thus the air is warmed, partly by the rays which come 
direct from the sun ; partly by those which are reflected 
from the earth ; partly by those emitted from the earth 
by the process of radiation ; but mainly by conduction 
from the warm earth's surface and the convectional rising 
of these warmed layers. Highlands are cooler than low- 
lands, largely because the air in these places is less dense 
than that nearer the sea level (Fig. 15). The presence or 
absence of large bodies of water very markedly modifies 
the effect of solar energy upon the atmosphere. As a 
result of these differences, the atmosphere is put in motion, 
winds are produced, clouds are formed, storms are started, 
and rains are caused. 

The movements of the earth in space also give rise to 
many variations in heat effect and atmospheric phenomena. 
As a result of the rotation of the earth, the greater part 
of its surface is lighted and warmed during a part of every 
twenty-four hours, and thus we have day and night. 

A second important movement of the earth is that of 
revolution, which causes the seasons (Figs. 8 and 17). 
Since the pole is inclined to the plane of revolution, the 
sun is made to appear to migrate in the heavens. During 



34 



PHYSICAL GEOGRAPHY. 



our winter, when the sun is vertical over that part of the 
earth which lies between the equator and the tropic oi 
Capricorn, the sun rises in the southern part of the heavens, 
and passes westward without rising high toward the zenith. 
Then in Arctic latitudes, the sun does not rise above the 
horizon ; and therefore in this region there is no alterna- 
tion of day and night. In the winter season, in temperate 






Fig. 17. 

Diagram to show the inclination of the sun's rays in different parts of the earth 
during the various seasons. Upper figure, spring and autumn ; right-hand 
figure, northern winter ; left-hand, northern summer. 



latitudes the journey of the sun across the heavens occupies 
a small fraction of the whole day ; and therefore in such 
regions the time during which the earth is receiving heat 
is less than the length of the night, during which almost 
none is received. 

Besides this fact of short days and long nights, the 
angle at which the rays reach the surface is much 



THE ATMOSPHERE. 35 

more oblique than in the summer season ; and before reach- 
ing the surface they are obliged to pass through a great 
thickness of atmosphere. These facts make the effect of 
the small amount of energy that does come, less apparent in 
winter than in summer, when many of the rays pass from 
a point near the zenith through a relatively small amount 
of atmosphere, reaching the surface more nearly at right 
angles (Fig. 17). After the sun has passed north of the 
equator, summer comes to the northern hemisphere, while 
winter prevails south of the equator. 

Thus at any point between equatorial and Arctic regions, 
there are two variations in the effect of the solar rays, one 
a daily and the other a seasonal variation. The tempera- 
ture of the air over the land normally rises during the 
day, and falls at night ; it rises in summer, and falls in 
winter ; and the amount of daily rising and falling is 
greater in summer than in winter. There is much variation 
in these respects according to latitude ; and there is less 
change in temperature between day and night, and between 
seasons, at the equator than in most other latitudes ; but the 
amount of heat received there is greater than in other parts 
of the earth. The greatest range in temperature, both sea- 
sonal and daily, is experienced in the higher latitudes. The 
least heat supply is received in polar latitudes; and here 
there is a great range between the summer and winter tem- 
peratures, but slight daily ranges, because in winter the sun 
does not rise above the horizon, while in summer it does 
not set. 

Moisture. — When rays of radiant energy enter a water 
body, they are in part transformed to "latent heat," being 
engaged in the process of changing the liquid to a gaseous 
condition. By this process of evaporation much of the 
energy exists in a form which is not apparent as heat so 



36 PHYSICAL GEOGRAPHY. 

long as the vapor condition lasts; but when the vapor is con- 
densed, this store of heat becomes apparent. Evaporation 
will take place even from a snow surface ; but the most 
favorable conditions for the production of water vapor are 
warm air in contact with a water surface. 

The capacity of the air for water vapor is limited ; and 
when no more can be contained it is said to be saturated. 
When there is little vapor in the air it is constantly capable 
of taking more until the limit of saturation is reached. We 
commonly say that dry air can absorb vapor. 1 If the amount 
of water upon the land is slight, the air in these places 
remains dry ; but naturally this cannot be the case with air 
over bodies of water, for there the conditions favor satu- 
ration. In the interior of continents, and in the upper 
layers of the atmosphere, there is the smallest proportion 
of water vapor. If the air from these places reaches the 
oceans, it may bring to them conditions of dryness, which, 
however, are soon changed to relative dampness. With the 
air in movement, saturation is less liable to occur than 
would be the case if the air were quiet. Therefore winds 
favor evaporation by bringing fresh supplies of air, and for 
the same reason they tend to prevent saturation. 

The capacity of air for water vapor also depends upon its 
temperature. A layer of air which is saturated at the tem- 
perature of 50° becomes relatively dry if its temperature is 
raised to 90°; and an air layer which is nearly saturated at 
90° will be obliged to give up some of its water vapor if the 
temperature is lowered a number of degrees. This is a very 
important point in the formation of clouds, storms, and rains. 
The actual amount of water vapor in the air represents its 

1 Strictly the air does not absorb vapor, but the water vaporizes regardless 
of the presence of the air. However, it is convenient to speak of the capacity 
of the air for water vapor, especially as the air determines the temperature. 



THE ATMOSPHERE. 



37 



absolute humidity ; but this is not a very important factor, 
because the same amount of vapor in air of different tem- 
peratures will produce very different effects. 

The point of greatest importance is the relative humidity, 
which is the percentage of water vapor actually contained in 
the air compared with the amount which the air at that tem- 
perature could contain if it were saturated. Thus the relative 
humidity of saturated air at a temperature of 60° is 100 per 





MONDAY 


TUESDAY 


WEDNESDAY 


THURSDAY 




6 XII 6 


6 XII 6 


6 XII 6 


6 XII 6 












luu 

80 
60 
40 
20 


/^ 




^\l 




AlA" 


1 




I 


w 


1 






J' 






1 












1 




' 


! 







| 


! 


1 


! 



SEP. II 



13 



14 



Pig. 18. 



Diagram showing daily change in relative humidity as a result of the daily 
change in temperature at Ithaca, N.Y. 



cent, for at that temperature no more can be contained ; 
but if the temperature is raised a few degrees, the air 
becomes capable of containing more water vapor, and the 
relative humidity is then less than 100 per cent. The tem- 
perature at which air containing a given amount of moisture 
becomes saturated is known as the dew 'point, for then vapor 
must be condensed. After a warm and apparently dry day, 
dew may be formed at night merely by lowering the tempera- 



38 



PHYSICAL GEOGRAPHY. 



1000 FT. \ Saturated. 



ture of the air, and thus increasing the relative humidity, with- 
out any change whatsoever in the absolute humidity (Fig. 18). 
It follows from this that there must be very marked differ- 
ences in the amount and effect of water vapor contained in 
the air. Over the oceans, the relative humidity is great, and 
the air nearly always near the point of saturation ; in the 
tropics, where the temperature is high, the absolute humidity 
is high, because warm air can contain much vapor ; and on 
mountain peaks, where the temperature is 
low, the amount of vapor is slight, because 
cold air has little capacity for water vapor. 
If the dry upper air descends to the earth, 
its absolute humidity is low ; and even if it 
commenced its descent in a saturated con- 
dition, its relative humidity decreases be- 
cause the temperature rises (Fig. 19) ; and 
if air currents move from cooler to warmer 
latitudes, their capacity for vapor is con- 
stantly increasing, because they grow con- 
stantly warmer and have a 
greater power of absorbing 
vapor. When they move 
. from warm to cooler regions 

ture of descending air. Starting in a their relative humidity in- 
saturated condition with a temperature , , . 

of 40° at 1000 feet, it reaches the surface creases, Decause tUeir tem- 

with a higher temperature and its ca- perature descends ; and 
pacity for vapor increased, while its . 

relative humidity has decreased. The when air rises Over land 

reverse takes place with ascent. elevations, or vertically by 

convection, the relative humidity is also increased, because 
air cools by expansion as it ascends ; and under such condi- 
tions the vapor is often condensed in clouds and rain. 

As a result of these varying conditions we get many varia- 
ble phenomena. Where the winds are prevailingly dry, and 




capacity for vapor 

considerably 

increased. 



THE ATMOSPHERE. 39 

the relative humidity low, desert conditions result; and 
where moist winds rise over rapidly ascending lands, condi- 
tions of excessive rainfall are produced. With air prevail- 
ingly dry, evaporation is rapid, while in regions of great rela- 
tive humidity, evaporation is slow and small in amount (Fig. 
60). Since water vapor contains a store of " latent heat " 
great stores of heat energy are transported from one latitude 
to another by the movements of vapor-laden air currents. 

Pressure. — The air, though so light and apparently almost 
without substance, actually has weight. At the seashore, 
the average weight of the air column is 15 pounds to the 
square inch ; but as we ascend into the air, whether in a 
balloon or on a mountain, the pressure of the air becomes 
less and less. Aside from this difference in air pressure the 
weight of the column of atmosphere at any single point is 
almost constantly changing. This is due to the fact that the 
air is very elastic and is subjected to a complicated series of 
movements. We shall be better able to understand the 
causes for these changes in pressure, and their effects upon 
the atmosphere, after we have examined in more detail the 
subjects of air temperatures and circulation. 

Effect of Gravity. — In a measure heat and gravity are in 
conflict in their effect upon the air. Heat is always expand- 
ing, some portions more than others, but gravity in trying 
to hold the air to the earth attracts the cooler and therefore 
denser parts more strongly than it does the lighter warmed 
portions. This starts a movement of the air, for the denser 
portions are drawn down to the surface and the lighter parts 
pushed above it. Gravity is thus a most important factor 
in determining the equilibrium of the atmosphere ; for its 
constant tendency is to restore an equilibrium which other 
causes are tending to destroy. 

Effect of the Earth's Rotation. — As the air moves in the 
form of winds or currents, there is a constant tendency to 



40 



PHYSICAL GEOGRAPHY. 




be deflected to one side, as a result of the effect of the 

earth's rotation. • This not only tends to turn the currents 

of air, but its influence is also felt in the ocean currents. 

In the southern hemisphere 
the currents are deflected 
toward the left, and in the 
northern hemisphere toward 
the right ; and we common- 
ly speak of the latter as the 
right-hand deflection (Fig. 
20). 

By revolving an orange 
or a ball around an axis, one 
can see that the motion at 
the equator is much more 
rapid than that at the poles, 
revolution carries 
every point along a circle, 

but the diameter of the circle decreases toward the pole 

(Fig. 21). Therefore in the course of a revolution a point 

near the equator travels 

a much greater distance 

than one near the pole. 

To do this, it must go 

faster, since the same 

period of time is al- 
lowed for revolution in 

any latitude. At the 

equator the rate is 1521 

feet a second, while near 

the poles the rate is 

greatly reduced. 

A body, no matter what its direction of movement (north, 



Fig. 20. 

Diagram to show how the moving currents np„ „!, 
are deflected from a straight line N-S 



70 




















/ 








\ 


.y 










\ 


36°, 






























26°, 






V 










\ 


I 




"" 








I 
















1 


f 
















to' 



Fig. 21. 

Diagram illustrating the decrease in diametei 

on different latitudes. 



THE ATMOSPHERE. 41 

east, south, or west), upon moving over the surface of a 
rotating sphere is constantly subjected to deflection, toward 
the right in the northern, and the left in the southern hemi- 
sphere. The amount of this deflection depends in part upon 
the velocity of the moving body, a rapidly moving current 
being less deflected than one with slow movement. It is, 
moreover, more decided in the higher latitudes than near the 
equator, reaching a minimum at the equator. 

So a body, such as a current of air or of water, moving 
southward in the northern hemisphere is turned to the right 
at a rate which will vary with its velocity and with the lati- 
tude. Hence it becomes turned toward the west. If the 
movement be toward the north, the right-hand deflection 
turns the body toward the east. In the southern hemi- 
sphere the turning is just opposite. That is to say, a body 
moving northward, or toward the equator, is turned toward 
the left or the west, and if moving away from the equator it 
is turned toward the left, or the east. 

Unfortunately the reason for this important principle seems 
incapable of clear non-mathematical expression ; at least, all 
such explanations known to the author are either mathe- 
matical, or incorrect or obscure. It therefore seems best for 
the present to leave the matter with a mere statement of the 
fact. 1 The fact, however, should be clearly understood, for 
upon it depends much of the matter which follows. It will 
be seen that the ocean currents and winds are thus deflected ; 
and upon this depends many peculiarities of climate in vari- 
ous parts of the world. 

1 The teacher will find the subject fully discussed in Ferrel's Popular 
Treatise on the Wind, referred to on page 84. A simple experiment illus- 
trating the principle may be tried by means of a circular table top which can 
be revolved. A marble dipped in ink, and allowed to run over the revolving 
table, will be deflected from a straight course in a direction varying with the 
direction of rotation of the table. 



42 PHYSICAL GEOGBAPHT. 



REFERENCE BOOKS. 

See also references at the close of Chapters III. -VII. 

Davis. — Elementary Meteorology. Ginn & Co., Boston, 1894. 8vo. 
$2.70. (Almost all points thoroughly treated in the light of the best 
modern knowledge.) 

Loomis. — Treatise on Meteorology. Harper Brothers, New York, 1870. 
8vo. $1.50. 

Scott. — Elementary Meteorology. Scribner, New York (Agents). Fifth 
edition, 1890. 12mo. $1.75. 

Tait. — Light. Macmillan & Co., New York (Agents). Second edition, 
1889. 8vo. $2.00. 

Capron. — Aurora. E. & E. N. Spon, New York (446 Brown St.), 1879. 
4to. $17.00. 

Guillemin (translated by Thompson). — Electricity and Magnetism. 
Macmillan and Co., New York, 1891. 8vo. $8.00. (Much on atmos- 
pheric and terrestrial electricity and magnetism.) 

Maxwell. — The Theory of Heat. Longmans, Green & Co. Tenth edi- 
tion. (Edited by Lord Eayleigh.) 1892. 12mo. $1.50. 

lyndall. — Heat as a Mode op Motion. Appleton & Co., New York. 
Fourth edition, 1883. 12mo. $2.50. 

In most good books on physics, the subjects of heat, light, and electricity 
are well treated from the physical standpoint. 

The American Meteorological Journal (monthly, Ginn & Co., Boston) 
contains a record of the progress in the subject, and many original articles 
of general interest. $3.00 a volume ; eleven volumes published. The publi- 
cation of this magazine has now (May, 1896) been suspended ; but in the 
various published volumes there is much of value. 



CHAPTER III. 

DISTRIBUTION OF TEMPERATURE. 

General Statement. — If nothing were present to interfere 
with or to distribute the solar rays that come to us, we 
should have a very regular distribution of heat over the 
earth's surface. At the equator the temperature would 
be extremely high, much higher than at present ; in the 
Arctic latitudes there would be very low temperatures ; and 
between these two belts there would be intermediate condi- 
tions. In each of these belts there would be seasons, and 
the difference between the day and night as at present^ 
This theoretical distribution of the solar heat is in reality so 
well defined that we are able to divide the earth's surface 
into three great climatic zones, — the Arctic, Temperate, 
and Tropical belts (Fig. 65~). 

In each of these zones there is a regular normal variation 
in the temperature of the different seasons, there being a 
gradual rise from winter to summer which with the corre- 
sponding descent from summer to winter makes what we may 
call the seasonal range or curve (Fig. 24). By the rise of tem- 
perature during the day, and its fall at night, a daily curve 
is also produced (Figs. 22, 27-29, and 33) ; and therefore the 
seasonal curve is made up of a large number of daily curves 
(Fig. 23). Theoretically, these should all be regular, and 
season after season we should have an almost exact repetition 
of these curves. However, in reality, this is far from being 

43 



44 



PHYSICAL GEOGRAPHY. 



Fahr. M 
100° 



the case ; and the divergence from the theoretical is due 
to the presence of a number of disturbing influences. These 
are (1) the effect of atmospheric movements, (2) the in- 
fluence of the oceans, or the absence of such influence, 

(3) the effect of topography. 
Effect of Atmospheric Move- 
ments. — This subject is 
again referred to in the 
chapter on winds, and now 
we need only consider a 
few of its general features. 
There is a regular circula- 
tion of the atmosphere, and 
numerous other movements 
which we may call irregular. 
Certain winds blow with 
moderate steadiness toward 
the equator, where the air 
rises and then flows away 
at a considerable elevation 
above the earth's surface. 
By this means much of the 
heat which reaches equatorial 
FlG - 22 - regions is borne away and 

Daily temperature curves. Lowest, Arc- distributed ill other zones, 
tic (winter) ; second, north temperate 

land interior (winter) ; third, same In the north temperate lati- 

(summer); fourth sea coast near the tudeg th general mov ement 

tropics (winter) ; fifth, same (summer). ° 

of the atmosphere is toward 
the east ; and this brings to west coasts the warm air from 
over the oceans, while to the eastern parts of continents, air 
is brought from the interior regions. By means of these 
and other general influences of the atmospheric circulation, 
the temperature of the earth's surface is greatly modified. 







DISTRIBUTION OF TEMPERATURE. 



45 



Smaller movements do locally what these great movements 
do in a general way. Thus a storm passing across the 
country brings conditions of cloudiness and rain, and pro- 
duces winds which are sometimes warm and sometimes cold. 
By this means air is sometimes drawn from cold, snow- 
covered lands ; or it settles from the upper cold layers of 
air ; or it may be drawn from the equable ocean. At the 
seashore, during the summer, the cool sea breeze may blow 
and modify the heat of the hot summer day (Fig. 38). 





April 


May 






































fi(f 














ttP 












1,0° 










30° 
90 
















10° 










0- 











■100 
00° 
80° 
70° 
60° 
50° 
h0° 



Fig. 23. 

Diagram illustrating mean seasonal rise in temperature, with daily and irregular 
changes superimposed. 



Influence of Oceans. — The ocean, and even large bodies 
of fresh water, are important modifiers of climate. As we 
have already seen, the ocean water warms very slowly, and it 
cools with almost equal slowness. Therefore the difference 
between the temperature of day and night, and summer and 
winter, is much less there than on the land, which warms 
rapidly during the summer day and cools readily at night 
and in winter. Over the ocean, in tropical latitudes, the 



46 PHYSICAL GEOGRAPHY. 

temperature range throughout the year is very slight ; and 
in temperate latitudes, while the range is much greater than 
this, it is still small compared with the range on the land 
(Fig. 24). Therefore near the seashore, the temperatures of 
the summer and the day are relatively low, while the tempera- 
tures of winter and the night are relatively high. Even on 
the shores of small lakes this influence of water is noticeable. 

On those coasts which are reached by prevailing winds 
from the ocean, as on the west coast of the United States, 
the general temperature is high, and the climate equable. 
Even in a short distance the temperature difference may 
be very marked ; and while on the shore the effect of the 
ocean is plainly felt, this influence becomes very much 
less marked at a distance of a few miles from the coast. 

Another very important influence of the ocean is that 
caused by the fact that this body itself is in motion. Both 
warm and cold ocean currents move on the surface of the 
sea and tend to equalize the temperatures of different parts 
of the earth. By this circulation, lands that would other- 
wise be uninhabitable have their climate rendered much 
more equable than that of regions in lower latitudes where 
these conditions of oceanic circulation do not exist. One 
of the best illustrations of this is the difference between 
the climate of Western Europe and Eastern America. 

As a general statement it may be said, that under the 
present conditions of distribution of land and water, ocean 
and air circulation, and alternation of day and season, the 
general climate of the globe becomes progressively colder 
as the polar regions are approached ; and as we pass 
from the seashore toward the interior of continents, we go 
from regions of equable climate, to those possessing great 
ranges in temperature between the winter and summer, 
and day and night. 



DISTRIBUTION OF TEMPERATURE. 47 

Effect of Topography. — It would be quite impossible to 
enter into this subject in much detail. In general, valleys 
are warmer than hilltops, partly because they are protected 
from the wind, and partly because the solar rays that fall 
upon the valley sides are in some degree reflected into the 
valley. The sides of hills, or of mountains which face 
toward the sun, are warmer than the north-facing sides ; 
and this is often very well shown in the natural distribution 
of plants, which rise higher on the southern side of the hill 
than on the northern side, where the temperature is less 
favorable to their existence (Fig. 68). 

Next to latitude, altitude is probably the most important 
feature in determining climate. If the elevation be suffi- 
cient, conditions in some respects resembling those of the 
Arctic climate may be found even under the equator. At 
a height of from 15,000 to 18,000 feet above sea level, 
vegetation ceases to exist, and perpetual snow covers the 
mountain tops. This is due to several causes, the most 
important of which is the fact that the air at great eleva- 
tions is less dense (Fig. 15), and hence cooler. Through 
this relatively thin layer, which is clear and free from large 
quantities of dust particles and water vapor, the rays that 
fall upon the surface are readily radiated into space. 

This illustration is interesting, since it shows that in 
the same latitude, and consequently with the same amount 
of solar energy, the two opposite extremes of tropical and 
Arctic climates may result. It brings out very strongly the 
fact that the mere amount of energy received does not 
determine the temperature of a place ; the subsequent be- 
havior of this is equally important. This same fact is 
shown by the difference between the climates of the sea- 
shore and the land at different places in the same latitude. 
Almost everywhere on the earth the influence of topog- 



48 PHYSICAL GEOGRAPHY. 

raphy upon temperature is shown, sometimes in great differ- 
ences extending over wide areas, again very locally and in 
small amount. Mountain ranges prevent the passage of 
vapor-laden air into the great enclosed basins, where dry 
clear skies exist, and where desert conditions are conse- 
quently produced ; and we might find many instances, great 
and small, to illustrate the influence of land forms upon the 
distribution of temperatures. 1 

Seasonal Temperature Range. — From the above, it is seen 
that latitude is no true indication of temperature ; for it is 
but one of several factors which tend to determine climate. 
However, it is one of the most important of the factors, and 
in general the temperature decreases from the equator 
toward the poles. Still, owing to the disturbing influence 
of the other factors, this decrease is not regular ; and hence 
the lines of equal temperature, or the isotherms, are not 
parallel to the lines of latitude, but often diverge very 
widely from them. If we examine the charts of isotherms 
(Plates 2, 3, and 4), we find that they are irregular, and that 
the irregularities vary with the season. Moreover, any 
given line, such for instance as the 50° isotherm, is in 
a different place in the opposite seasons. In other words, 
I the temperature of every part of the earth changes with the 
season ; but the change is different in amount in different 
places. 

This seasonal change may be called the temperature 
range or curve. If the temperature changes of any given 
region are plotted upon a diagram, in which both the 
months and the scale of degrees are shown (Fig. 24), we 
find that there is a gradual rise in the spring to a time 
after midsummer, when the temperature falls until after 

1 Many of these features are illustrated in the accompanying isothermal 
charts. 



DISTBIBUTION OF TEMPERATURE. 



49 



midwinter. Year after year this is true, though each year 
will show a slight difference from those which precede 
and follow. Even in different regions the same is shown ; 
but there is much variation in the form of the seasonal 
curve of different places. Such a curve shows how much 
difference there is between seasons, and when it occurs. 
We find that the height to which the temperature rises 




Fig. 24. 
Seasonal temperature ranges. Constructed to have northern and southern sum- 
mer coincide. Hence for southern hemisphere June should read January, etc. 

in the curves is very variable in different parts of the earth, 
and the same is true of the length of the warmer or the 
colder part of the curve, which is the same as saying that 
the length of the warm season differs in different places. 

If we plot such a curve as this for a place over the ocean, 
we find that it is relatively flat, because the difference 
between the winter and summer temperatures is not very 



50 PHYSICAL GEOGRAPHY. 

great. On the other hand, in the central parts of continents, 
where the winter is relatively cold, and the summer warm, 
the curve rises to a much greater height. At the equator, 
the curve is much flatter than in temperate and Arctic 
latitudes, where the difference between summer and winter 
temperatures is great. In any one of these zones there 
may be marked differences even in neighboring places. 

Upon examining one of these seasonal curves, it will be 
noticed that the time when the temperature is highest does 
not correspond with the period when the greatest amount 
of heat is received from the sun ; nor is the coldest time of 
winter coincident with the shortest days. In other words, 
there is a lagging, and this is due to the cumulative effect 
of the heat or cold. In the early summer, the ground 
is still cool from the effects of the last winter, and in high 
latitudes there is still snow upon the ground. It takes some 
time for the sun's rays to warm the ground and the air ; 
and when this is done, the effect of solar energy becomes 
greater than before, even though the days be shorter and 
the amount of energy coming from the sUn less than in mid- 
summer. In the opposite season, the effect of radiation 
during the long nights becomes most marked after the 
middle of winter, which is really the 22d day of December. 
Therefore January is almost invariably colder than Decem- 
ber, and February also may be colder than December. 

For the sake of diagrammatic illustration, the seasonal curve 
is represented as being a continual rise and fall of tempera- 
ture. It represents the average temperatures for the several 
parts of the different months. In reality there is no such 
regular and uniform rise, but it is interrupted by daily 
risings and fallings (the daily curve, pp. 60-62), and by 
irregular interruptions (Fig. 23). For days at a time the 
normal seasonal rise or fall may be interrupted, and even be 




Face page 50, 



Isothermal < 




>r the year. 



DISTRIBUTION OF TEMPERATURE. 



51 



replaced by a temporary descent (Fig. 25). This happens in 
our latitude when storms or cold waves pass over us, and pre- 
vent the effect of the sun's heat from becoming apparent. 
Thus in winter we may have thaws, or in midsummer the heat 
may be tempered by several days of cool weather ; but there 
are more irregularities during our winter than during the 
summer. The temperature curve shows only the average 
of these, its chief value being to illustrate the effect of 
the sun's rays as the season changes, and to show how differ- 
ently this effect is manifested in various places. 





DAILY MEAN TEMPERATURES OF THE STATE FOR 1891, WITH NORMAL VALUES 




90 

80 

70 

f 60 

m 50 

£ 40 

° 90 

20 

10 










„L. | -UCST SE P«-«» OCT0U. .0....I. 




90 
SO 
70 
60 
60 
40 
SO 
20 
10 










_j_i » ,1 11 j ;> ,7 |7.| 7 .7 1,| * 








































































































































































" 








































































































































































" 


10 


*• 


60 75. 90 1H 


HI 


US. lit 


DAY OF THE YEAR 





Fig. 25. 

Seasonal curve for New York state. Irregular variations shown by 

the lighter line. 

Isothermal Charts. — The best graphic way to show the 
distribution of temperature over the earth, is by means of 
isothermal charts. The isotherm is the line of equal tem- 
perature ; and the chart may show these lines for the day, 
or for the month, or for the year. If for the year, they 
represent the average of all the temperatures during that 
time ; or if for the month, the same average for day and 
night throughout the month. Every place which has the 
same average temperature for the period represented on 
the chart, has the same isothermal line. That is, if the 



52 PHYSICAL GEOGRAPHY. 

average temperature for a given month is 50° at London, 
Boston, Buffalo, etc., the 50° isotherm for that month is 
made to pass through each of these places. 

On the isothermal chart which shows the average tern- 




Fig. 26. 
Isotherms for February, 1878-1887. 



perature for the year (Plate 2), it will be noticed that in 
general the temperature decreases from the equator toward 
each of the poles ; but in each hemisphere there are numer- 
ous exceptions (Fig. 26). The rate of decrease is very 



DISTRIBUTION OF TEMPERATURE. 53 

variable in different latitudes. While there is a general 
tendency for the lines of equal temperature to run parallel 
with the lines of latitude, at times the divergence is so 
great that the isotherms extend in a north and south 
direction. There is much less irregularity in this respect 
in the southern than in the northern hemisphere ; and this 
is easily explained by the fact that the land is mostly in the 
northern hemisphere. One is able to see the disturbing 
influence of the land in many places. 

Another effect of the greater abundance of land in the 
northern hemisphere, is that the line of greatest heat, or 
the heat equator, is north of the true geographic equator. 
The land becomes much warmer than the ocean, and hence 
the highest temperatures are found in the interior of conti- 
nents. This is not because more energy is received, but 
because the amount that does come is much more effective 
in warming the land and the air. Since radiation proceeds 
more readily from the land than the water, the average 
temperatures in northern regions are lower than in the 
southern hemisphere. Other general influences are notice- 
able upon the chart of annual isotherms. For instance, in 
the northern Atlantic, where the warm Gulf Stream extends 
toward the Arctic circle, the isotherms are bent northward ; 
and along the eastern coast of the United States, where the 
cold Labrador current flows close to the continent, and 
isothermal lines are bent southward. 

On the western side of North America, the influence of 
the prevailing winds is well shown where they blow from 
the warm Pacific upon the coast. This is particularly well 
illustrated on the isothermal charts of the United States 
(Plate 3), where we see a very marked difference in the 
temperature of the east and west coast. Thus there is 
a great range in temperature between Key West, on the 




Face page 35. 



Isothera I " 




H'or July. 



DISTRIBUTION OF TEMPERATURE. 55 

extreme southern end of Florida, and the northern part 
of the coast of Maine, while in the same distance on the 
west coast the temperature differences are much less. From 
Key West to Cape Hatteras the influence of the warm 
Gulf Stream is felt, while on the New England coast 
the temperatures are lowered by the cold Labrador current ; 
but on the Pacific coast the influence of the warm ocean 
is manifest from Southern California to Washington. A 
study of the charts will show many other variations in the 
isotherms. 

In the isothermal charts which represent the typical sum- 
mer and winter conditions, similar phenomena are noticed ; 
and in some cases they are more strikingly illustrated than 
on the annual chart. The heat equator of July (Plate 4) 
follows the sun well up toward the tropic of Cancer, but 
it does not follow the sun as far when it takes its southern 
journey during our winter ; and in the Atlantic, where there 
is much more neighboring land, the migration of the heat 
equator is more marked than in the broad Pacific. We notice 
also that the influence of the Gulf Stream in deflecting the 
isotherms is more important in January than in July, 
when the neighboring ocean waters are themselves warmed 
by the summer sun. 

In the two hemispheres there is also a difference in 
in the amount of migration of the isotherms for the lower 
temperatures. In the southern hemisphere the isotherm 
of 50° in July barely reaches Africa and Australia, and its 
position in January is not greatly different (Plate 5). 
This shows the influence of the prevailing condition of 
water in that hemisphere ; and the same fact explains the 
general parallelism of this isotherm with the lines of lati- 
tude ; but in the northern hemisphere, where there is 
more land, the isotherm of freezing in July is in the Arctic 



56 PHYSICAL GEOGRAPHY. 

circle, while in January it extends below the 40th parallel in 
several places. The isotherm of 50° migrates from northern 
Scandinavia, Iceland, and Labrador, in July, to Spain and 
the Carolinas in January. 

In the higher latitudes of the northern hemisphere, the 
influence of the land is shown by the fact that in January 
excessively low temperatures occur in the interior of conti- 
nents. Thus so far as we know, the coldest parts of the 
earth are in these continental interiors, such as Asia. The 
winter " cold pole " of the world is not found high up in the 
Arctic latitudes, but in central Siberia near the Arctic circle 
(Plate 5). This is due to the fact that in these dry land 
interiors, radiation causes excessive cold during the long 
winter night. It is possible that when the Antarctic conti- 
nent or the interior of Greenland are better known, we may 
find upon these snow-covered lands even lower winter tem- 
peratures than those of northern central Asia. 

On the January and July charts of the United States 
(Plates 6 and 7), we find the greatest difference in tem- 
perature in the dry interior regions of Dakota and Mon- 
tana, and the least at Key West and on the southern coast 
of California, where the equable ocean waters prevent 
either excessively high summer temperatures, or excessive 
cold in the winter. Another place where the temperature 
of the United States is subjected to a great range in the 
different seasons, is in the desert region of the Great 
Basin. Here the sun's rays of the summer day readily 
pass through the dry, cool air and raise the temperature 
of the ground, and the lower air layers, to a very high 
degree. At night and in the winter, radiation proceeds 
with rapidity, because the air is clear and offers little ob- 
struction to the passage of the radiant heat ; and therefore 
in the winter nights the temperature becomes very low. 




Face page 56. 



■Id 0-20 f 



dc'30 



--^-t^irri 




r January. 



DISTRIBUTION OF TEMPERATURE. 



59 



The influence of topography is also well shown in several por- 
tions of the charts for the United States, and also on the New 







York chart (Plate 8), where the isotherms are seen to extend 
up the valleys, showing that they are warmer than the hills. 



60 



PHYSICAL GEOGRAPHY. 



Daily Temperature Curve. — The daily curve represents 
for the day what the seasonal curve does for the year. It 
shows the rise in temperature during the daytime and its 
fall at night (Fig. 27). Unless interfered with by some 
accidental cause, the temperature rises from sunrise till 
early afternoon, and then descends until late in the night. 
As in the case of the seasonal curve, 
| . | the time of highest temperature is not 

I i I : 5 I when the sun's rays are strongest, nor 
is the coldest part of night at midnight. 
The explanation is the same, the heat of 
the sun in the morning being partly ex- 
pended in warming the earth which was 
cooled in the preceding night ; and the 
temperature at night time continues to 
descend after midnight, because the radia- 
tion of the heat that came during the day 
proceeds uninterruptedly, and its influence 
is not checked until the sun again rises. 

There is much variation in the daily 
curve in different latitudes (Fig. 22), and 
even in different places in the same 
latitude. The daily change in tempera- 
ture is relatively slight on the seashore, 
and very great on the land ; and the range is much 
greater in temperate latitudes than in the tropics. In the 
Arctic regions, where the sun is above the horizon in the 
summer and below it in the winter, the daily curve is of 
very little importance, and may be entirely masked by acci- 
dental causes. 

Since in many parts of the earth there is a great variation 
in the length of day and night during the different seasons, 
the daily temperature curve varies with the season. Thus 







r 


V 








\ 








\ 




































/ 


/ 


\ 


^ 


J 







Fig. 27. 



A normal daily range 
for summer and for 
winter in New York. 



DISTRIBUTION OF TEMPERATURE. 



61 



in our latitude the temperature rises much higher in summer 
than in winter (Fig. 27). 

While normally the temperature curve is that which has just 
been described, in reality it is subjected to many variations 
and interruptions (Fig. 28). The tendency is for the tern- 



- 


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4li"r i " 9 to 11 13 14 15 16 17 18 18 20 MAY 21, 

MAYS TEMPERATURE 1893 

ITHACA 



Fig. 28. 
Normal daily curve followed by an interruption of several days. 

perature of the day to rise above the average for that season, 
and to fall below it at night (Fig. 23). Oftentimes the 
daily curve is so changed (Fig. 29) that instead of a rise 
during the daytime, we have a fall in the temperature 
(Fig. 64) ; or the temperature may continue to rise through- 



,0. 








































































































1 
























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DEC.27, 
1892 


28 29 


30 3 


1 JAN. 1,1893 2 3 4 6 6 JAN. 7,1893 

TEMPERATURE 

ITHACA 



Fig. 29. 

Daily temperature record, showing interference with the normal rise 
and fall of temperature. 

out the night, the opposite of what would normally be the 
case. Cold waves or storms are often the causes for these 
changes, and many local and temporary effects may thus be 
produced. The presence of clouds, or of much, moisture 
in the air, or of winds from the ocean (Fig. 39), may very 



62 



PHYSICAL GEOGRAPHY. 



effectually modify the normal daily rise and fall of tempera- 
ture. 

Temperature Ranges. — The study of the isotherms of 
a region gives us an idea only of the average temperatures 
of different places. In a study of climate it is necessary 
to know something of the changes in temperature, both with 
reference to the amount (Fig. 30) and the rate. 




Fig. 30. 
Temperature ranges in the United States in degrees Fahrenheit, 1892. 



No better illustration can be found of the differences that 
may exist between places on the same isotherm, than that 
of St. Louis and San Francisco, which are on the same annual 
isotherm (55. 7°) and on nearly the same parallel of latitude. 
In San Francisco, the average for September, the warmest 
month, is a little less than 60°, while the January isotherm 
is about 50°, the actual range between the averages being 
about 9.5°. At St. Louis, the January isotherm is 31°, while 



DISTRIBUTION OF TEMPERATURE. 



63 



the July isotherm is 78°, a range of about 47°. Taking the 
highest and lowest temperatures for each place, the differ- 
ence is even more striking, for we find a range of 61° in 
San Francisco, while in St. Louis the range is 128°. There- 
fore, though they are on the same isotherm, the climates of 
the two places are quite different. 

The lowest temperature ever accurately observed on the 
earth was less than —90°, the highest over 127°, the former 



30 X l/T/^klX^^^^^ 
/I //'-I i\H v VC/iA 1 ~\y\. 


\f 1/ 1 / 


4^W-70 
v. ^>C/* 
L,30 

sty 40 

7Q~50 


32^ 



Fig. 31. 
Minimum temperatures observed in the United States, 1892. 



in Siberia, the latter in Algeria. This is a range of nearly 
218°. Such extreme ranges are of course impossible in any 
single place ; but in some of the dry interiors of continents, 
very extreme temperature ranges are sometimes experienced. 
In Siberia, where the greatest ranges are found, temperatures 
181° apart have been observed ; and in the northwestern 
states of this country, ranges of over 150° have been meas- 
ured. On the other extreme, ranges of only 40° or less are 



64 



PHYSICAL GEOGBAPHY. 



observed at Key West and on the coast of California. In 
some of the tropical islands of the Pacific, the greatest differ- 
ence in temperature during the year is often not over 18° 
or 20°. More than half of our country experiences ranges 
greater than 100° (Figs. 30-32). 

If these temperature changes came slowly, their effect 
would not be so very difficult to endure ; but in places of 
great annual change, there is almost always great change 









S?f> %r. 




7 °f VA^H^PP^ v 


f/^' 








so i ^\v/ A Jv~^ /^^^^C^y 


\ — 1 










[ ;_ 


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y^M 




\W \ \ ^Xi^ 






V -95 




V \\ ^ / \V Q^^ 1 ^ 










1 \ x ^n\ x 7uo 






\J^90 





Fig. 32. 
Maximum temperatures observed in the United States, 1892. 

in short periods. In Montana (in December, 1880) in less 
than eighteen days, the temperature fell 117°, the thermome- 
ter on the 12th registering 58°, and on the 29th — 59°. In 
the greater part of northern United States we are accustomed 
to similar changes in winter, though they are very rarely so 
extreme as this. After a few days of moderate warmth 
during the unseasonable winter thaws, a cold wave spreads 
over the eastern states, and zero weather prevails, not un- 



DISTRIBUTION OF TEMPERATURE. 65 

commonly causing a drop in temperature of 60° or 70° in 
a few days. 

We are even liable to very excessive changes in a single 
day. Where the air is dry, as in parts of the arid regions, 
a change of 40° is not uncommon in the summer, as the 
result of the heat of the day, followed by the coolness of 
the night, which is caused by the radiation through the clear 
dry air. Near the ocean the difference of the day and night 
temperature is often very slight, particularly in the winter. 
At Key West the day and night temperatures differed only 
about 7° in December, 1877. 

Aside from these regular daily ranges there often occur ex- 
ceptional changes (Fig. 64). In winter a cold wind may fol- 
low a rain storm and cause the temperature to descend below 
zero with a change of 35° or 40° in a few hours. In this 
case the nocturnal radiation is an aid in the fall of tempera- 
ture. A daily change of 50° is not uncommon in Montana ; 
and in Texas the thermometer has been known to fall 63° in 
sixteen hours. It is said that in Thibet the temperature 
has fallen 90° in fifteen hours, or from 68° in midday to 
— 22° at night. In summer there are also great ranges ; but 
they are not so noticeable, nor are they so severe, as those 
which come at times in winter. 

Earth Temperatures. — At the very surface of the earth 
the ground is warmed when the sun's rays are present, and 
cooled when their effect is absent. Below a depth of a few 
feet the influence of the sun is not very noticeable, and from 
this point downward, the temperature of the earth is practi- 
cally permanent, and is determined by the heat of the interior. 

There is much difference in the effect of changes in tem- 
perature in different parts of the earth. At the equator the 
ground is very warm at the surface, and there is a slight varia- 
tion throughout the year. At the depth of five or six feet 



66 



PHYSICAL GEOGRAPHY. 



the intensity of the heat has decidedly decreased, and soon the 
zone of no variation is reached. In temperate latitudes, 
the difference between summer and winter temperatures is 
so great that the surface becomes warm in summer, and in 
winter cools down to temperatures lower than the freezing- 
point. In the winter, in such regions, frost exists in the 
ground often to a depth as great as six or eight feet. In 
the Arctic regions, where the sun's rays are of little power, 
and where radiation is excessive, the ground is often per- 
manently frozen to a depth of several hundred feet. During 
the summer the surface layers lose their frost and thaw, 

and plants grow over 
earth which is per- 
manently frozen. 

The ground is such 
a poor conductor of 
heat that it takes 
many weeks for the 
effect of the summer 
heat, or the winter 
cold, to reach to a 
depth of ten or fif- 
teen feet. Therefore 
at such depths the 
seasonal curve lags 
behind that of the 
air ; and at the same 
time the temperature 
range is less. 
At the very surface, the earth temperature changes more 
than that of the air (Fig. 33). This is because the earth 
readily absorbs heat and radiates it with almost equal rapidity. 
For this reason the ground at midday is warmer than the air, 




Fia. 33. 

Daily ranga at the surface of the earth (dotted 
line) and of the air ten feet above (heavy line). 
Ithaca, N. Y., July, 1893. 



DISTRIBUTION OF TEMPERATURE. 67 

while at night time its temperature is lower than that of the 
air. These facts of earth temperatures are important in 
explaining the heating or the cooling of the lower air layers. 



REFERENCE BOOKS. 

See also Buchan's memoir referred to at the end of the next chapter ; and 
also the books on general meteorology, notably those by Davis, Scott, Waldo, 
Greely, Abercromby, Blanford, Woeikof, Hann. and Croll. 

The Berghaus Atlas, volume on Meteorology (Hann, Atlas der Meteo- 
rologie. Justus Perthes, Gotha, Germany, 1887. 15 marks 1 ), although in 
German, contains many charts upon temperature distribution, etc., which 
will prove of value in the schools. 

The Annual Reports of the Signal Service, and now of the Weather 
Bureau of Washington, contain much information relating to the tempera- 
ture, wind, rain, etc. , of the country. 2 
Hazen. — The Climate of Chicago. Bulletin X, U. S. Weather Bureau, 

Washington, 1893. (Describes some interesting effects of the lake upon 

temperature. The other bulletins of this series are also of value.) 

1 Under the present law governing' the importation of foreign books no duty is charged on 
those in other languages than the English. Foreign books may be ordered direct from the 
publishers, or through some New York, or other importers. With all charges added, the mark 
becomes equal to about $0.25, the franc to about $0.21, and the shilling to about $0.25; but in 
the last case a duty may also be charged. While this does not give the actual price, it furnishes 
a close approximation. 

1 Where no price is given for government publications it indicates that they are distributed 
free of cost ; but in many cases all of the copies are exhausted, and the only way to obtain them 
is from the large city second-hand bookstores. Sometimes it is not possible to obtain 
government publications without the aid of a congressman ; but this will be easily obtained 
by most schools. 



CHAPTER IV. 

GENERAL CIRCULATION OF THE ATMOSPHERE. 

General Statement. — Since the air is very elastic, and its 
condition easily changed by variations in temperature, it is 
readily caused to move. No better illustration can be found 
of the mobility of the air under these circumstances, than 
that which is so often noticed on heated deserts. The ground 
becomes warmed, the air is heated by contact with it, and 
this causes the air to expand and become lighter so that a 
tendency to rise by convection is produced. Soon this ten- 
dency becomes so strong that the lower air must move up- 
ward, thus starting a dust whirl on the desert. The move- 
ment thus started by the effort of the denser air to take the 
place of the warmed layers causes very violent, though very 
local, winds. In a room, a warm stove, lamp, or an open- 
grate fire, causes the air to move, and starts a circulation 
which is often very noticeable. 

The reverse process of cooling the lower air layers causes 
a condensation which necessitates a settling down of other 
air. We may often see an illustration of this on a cold 
winter night when the air is quiet. If the window in a 
warm room is then opened, the cold, dense outside air flows 
in, producing a very perceptible current. 

If in place of these local illustrations, we substitute large 
areas of the earth's surface, we find an explanation of many 
of the greater features of the atmospheric circulation. Over 
equatorial regions, the air is constantly being warmed during 
the day, and therefore expanded. Accompanying this ex- 



GENERAL CIRCULATION OF THE ATMOSPHERE. 69 

pansion, there is rising caused by the greater density- of the 
surrounding air, and so a circulation is produced which 
exerts its influence over a very large area. This circulation 
consists of four parts : (1) the inflowing surface winds, 
(2) the uprising currents, (3) outflowing winds at high 

N 




Ferrel's ideal diagram of the planetary circulation. Dotted arrows show upper 

currents of air. 

elevations, and (4) down-settling air at some distance from 
the equator. There are other features of this great circu- 
lation which we will soon consider. Similar winds upon a 
smaller scale are produced over continents, and even on the 
land along the seashore. 

When warm air is expanded and raised it pushes away the 
air above it, the barometric pressure is decreased, because the 
air column is lighter ; and when the air is cooled, it becomes 



70 



PHYSICAL GEOGRAPHY. 



denser, and hence the barometer registers a higher pressure 
of the air. Therefore the relation between air pressure and 
wind is very intimate ; and where, for any reason, low-pres- 
sure areas exist, winds are found blowing toward them 
(Plate 9). This is the case in certain areas which are 
permanently warmer than the surrounding regions, and also 
in those disturbances of the air which are classed as storms. 
A barometric gradient is produced, and the winds move as 
if they were going down grade. The air moves away from 
high and toward low pressure areas. 

Classification of the Winds. — For the sake of simplicity in 
the consideration of the movements of the atmosphere, it 
seems well to adopt some classification of air movements. 
The one here proposed is a logical division; but other classi- 
fications might be used, the only object of such a division 
being to group like kinds. 



Planetary or Perma- 
nent. (Due to plan- 
etary causes of a 
permanent nature.) 



Periodical. (Due to 
periodical causes.) 



Irregular. (Due to 
causes apparently 
of an irregular na- 
ture.) 



Trades. 

Anti-trades. 

Doldrums; or equatorial calms. 

Horse latitude winds and calms. 

Prevailing westerlies. 

J Migrating winds and calm belts. 

[ Monsoons. 

f Land and sea breezes. 

\ Mountain and valley breezes. 



Seasonal winds 



Diurnal winds 



Eclipse winds. 
Tidal breezes. 



Storm winds 



Accidental winds 



' Desert whirlwinds. 

Cyclonic winds. 

Anticyclonic winds. 

Thunderstorm winds. 

Tornado winds. 

Landslip blasts. 

Avalanche blasts. 
| Volcanic winds. 
L Waterfall breezes. 




Face page 70. 



Winds and isol 




'or January. 



GENERAL CIRCULATION OF THE ATMOSPHERE. 71 

Planetary or Permanent Winds. — There are certain winds 
whose force and direction depend upon the fact that there is 
a variation in the amount of heat received in different lati- 
tudes of the earth, and that the earth is rotating about its 
axis. These may be called planetary winds, because they 
would be developed upon any planetary body where similar 
conditions prevail. Or we may call them permanent, because, 
compared with other winds, their direction and force are prac- 
tically permanent. In some places they are greatly modified 
by other causes; but they are so strongly developed that their 
influence is felt all over the earth. They are, as it were, 
the general atmospheric winds ; and together they form the 
fundamental circulation of the atmosphere. They may be 
described under several headings. 

Trade Winds. — Since the air over the equatorial regions is 
warmed more than that on any other part of the earth's sur- 
face, the denser air moving in toward the warmer region 
causes currents. The equator may be fairly compared with 
a stove, over which air rises by convection, and toward which 
currents flow. These inmoving winds are called the trades, 
because they blow with marked permanency and steadiness ; 
and in planning their journey, whenever possible, vessels 
choose a course which will allow them to take advantage of 
the trade winds. These winds move toward the equator 
(see Plates 9, 10, and 11), but instead of blowing directly 
toward it, they are deflected by the effect of the earth's 
rotation. North of the equator their direction is from the 
northeast, while south of it they move from the southeast. 
They are much less well developed over the land than over 
the water ; and when they blow from the water to the land, 
they are often deflected because of its influence. Over the 
land they may even be destroyed. 

The air in the trade winds is moving from colder to 



72 



PHYSICAL GEOGRAPHY. 




Plate 10. 
General circulation of the Atlantic for July. 



GENERAL CIRCULATION OF THE ATMOSPHERE. 73 



7 7Z7(r n Z r ? 




Plate 11. 
General circulation of the Atlantic for January. 



74 PHYSICAL GEOGBAPHY. 

warmer regions, and therefore its capacity for water vapor is 
constantly increasing. Therefore they are drying winds, and 
when they blow over the ocean, evaporation is rapid, while on 
the land, where water vapor is not readily obtained, they pro- 
duce deserts in many places. Since the temperature of this 
air is high, when blowing over the ocean the amount of 
water vapor which it is enabled to carry is very great; and 
much rainfall is caused if the air is made to rise, as is the 
case when the trades blow upon rising coasts. 

Doldrum Belt. — Over the heat equator, the air in this great 
planetary circulation rises by convection ; and in this place a 
condition of almost permanent calm is produced (Plates 10 
and 11). This is particularly the case over the oceans, but 
over the land other causes may interfere. The doldrum belt 
is situated between the north and south trade winds, and it 
migrates from season to season as the heat equator changes 
its position. Since the air in this belt is warmed, it 
contains much water vapor, and it is a very rainy belt 
because this humid air rises by convection, and cools 
dynamically until the dew-point is reached. Therefore, 
during the day the sky almost invariably becomes cloudy, 
and rains fall. 

Anti-trade Winds. — The inflowing of surface air, and its 
uprising, makes necessary an outflow of air at a higher level. 
This outflowing air moves away from the equator, in either 
direction, and produces what is known as the anti-trades. 
These winds are not felt on the land, excepting on those rare 
peaks which rise to a height of 10,000 or 12,000 feet above 
sea-level. They move in a northeasterly direction in the 
northern hemisphere, and toward the southeast in the south 
ern, in each case being turned from a true north or south 
direction by the deflective influence by the earth's rotation. 
Their permanency is shown by the fact that the upper clouds 



GENERAL CIRCULATION OF THE ATMOSPHERE. 75 

move in these directions. This upper air movement con- 
tinues in the temperate latitudes. 

Horse Latitude Winds. — After traveling for a certain dis- 
tance from the equator, the air commences to settle, and some 
of it reaches the earth's surface near the poleward margins of 
the trade wind belts. These regions of settling air are known 
as the horse latitudes, and because the air is descending in 
these belts, the prevailing condition is that of calms or light, 
variable winds ; but these calm belts are not so pronounced 
as those of the doldrums (Plates 10 and 11). Over the land, 
the horse latitude belt is not so distinctly developed. 

Prevailing Westerlies. — Apart of the upper circulation of 
the anti-trades continues toward the poles; and because the 
polar regions are places of permanent low temperature, there is 
a tendency for the upper air to move toward them. These air 
currents are deflected to the right in the northern, and to the 
left in the southern hemisphere, so that in the upper latitudes 
there is a whirl of air known as the circumsolar whirl, which, 
in both hemispheres, produces a condition of prevailing 
westerly winds, both near the surface (Plates 10 and 11) and 
in the upper layers of the atmosphere. These whirls produce 
a condition of permanent low pressure in the polar regions; 
for there is an eddy produced, which is somewhat analogous 
to that formed by the escape of water from a bath tub. 

In the upper air the east-moving winds are remarkably 
permanent, as any one may see by watching the movements 
of the upper clouds. At the surface, the tendency toward 
the development of east-moving air currents is greatly inter- 
fered with by other causes. This is much more strikingly 
shown in the northern than in the southern hemisphere, 
where land is less abundant. In the latter hemisphere, sail- 
ing vessels may go around the earth with prevailing fair 
winds driving them onward (Plate 9). To do this, they 



76 PHYSICAL GEOGRAPHY. 

must go past Cape of Good Hope, and return by way of 
Cape Horn. In the northern hemisphere, the most striking 
influence of the prevailing westerlies upon the surface, comes 
from the fact that they determine the path of movement of 
the greater number of our storms. 

Periodical Winds. — There are certain changes of a period- 
ical nature, which tend to start the air in motion in a definite 
way ; and this tendency is repeated as these periods return. 
The most important of these changes are those which arise 
from the variation in supply of solar energy in the different 
seasons, and in the change from day to night. The periodi- 
cal winds may therefore be classed as seasonal and diurnal 
winds ; and in the group may also be included two minor 
classes of periodical winds, eclipse and tidal breezes. 

Seasonal Winds. — We get a large supply of heat in one 
season, and a very much smaller amount in the opposite 
season ; and the differences in seasons are very much greater 
far from the equator than they are in the equatorial belt. 
Therefore the seasonal effect upon the atmosphere, is less 
marked in the equatorial belt than elsewhere on the earth's 
surface. Still, even in equatorial regions, as the season 
changes, the movement of the sun in the heavens produces a 
very decided effect upon the atmospheric circulation. 

Migrating Wind and Calm Belts. — The wind charts 
of the Atlantic Ocean (Plates 10 and 11) show that there 
is a migration of the belt of calms as the season changes. 
The trade wind belts also move northward and southward ; 
and therefore in one season a region within the tropics 
may experience the calms of the doldrums, while in the 
opposite season, the dry and permanent trade winds blow 
steadily day after day. In the same way, a region situ- 
ated near the northward or southward margin of the trade 
wind belts, may in one season feel the trade winds, while 



GENERAL CIRCULATION OF THE ATMOSPHERE. 



77 




Fig. 35. 
Summer monsoons in India. 



in the opposite season the variable winds of the horse lati- 
tudes prevail. 

Monsoon Winds. — In latitudes outside of the tropics, where 
large land masses exist, very interesting winds are often caused 
by the difference in temperature between the land and. water. 
During the summer the land 
areas become warmed, and 
are covered by an area of 
permanent low pressure, to- 
ward which air currents move 
from all directions (Fig. 35). 
The air rises over the warm 
land, and its place is taken b}^ 
air from the relatively cool 
oceans. In the winter sea- 
son, the land becomes cooler 
than the water, and the air is caused to settle over the land 
and to move out from these areas of high pressure (Fig. 
36), toward the then relatively warm oceans (Plate 9). This 

class of wind, which is very 
pronounced in Asia, is known 
as the monsoon wind. Similar 
winds are noticed in other 
continents, and we now know 
this class of air movements as 
the monsoons. 

In Asia, the monsoon winds 
blow in toward the central 
regions, across India, China, 
and other countries. During 
the winter they blow in the opposite direction. In Australia 
and other continents, the monsoon system of winds is very 
well developed ; and on the Spanish peninsula, the same 




Fig. 36. 
Winter monsoons in India. 



78 



PHYSICAL GEOGBAPHT. 



tendency toward inflowing summer and outflowing wintei 
winds is quite pronounced. 

Even where the distinct monsoon condition is not pro- 
duced, a tendency to the production of this class of wind 
often expresses itself in the disturbance of the wind direc- 
tion. This is very well illustrated along the Texas coast; 
where the summer trade winds are deflected until they 
blow upon the land. This phenomenon is shown on Plates 

10 and 11 ; and on these 
charts it will also be noticed 
that in the winter, the pre- 
vailing winds of the coast 
of northern United States 
are from the land toward 
the ocean, while in summer 
their direction is much 
less definite. That is to 
say, the prevailing wester- 
ly winds are strengthened 
in winter and weakened 
in summer by the monsoon 
tendency, which in sum- 
mer is not sufficiently 
powerful to entirely invert 
the prevailing westerlies. 
In cold regions, such as Greenland, there is a tendency 
toward the production of outflowing winds in summer, 
because the snow-covered land is colder than the water. 
Another continental effect, not, however, dependent upon the 
temperature differences, is the retardation of winds in their 
passage over the land. As a result of friction, the winds of 
the land are less violent than those of the water; and mountain 
ranges may effectually check the winds and destroy them. 




Fig. 37. 
The sea breeze. 



GENERAL CIRCULATION OF THE ATMOSPHERE. 79 



Diurnal Winds : Sea and Land Breezes. — Since the heat of 
the day is followed by the coolness of the night, the atmos- 
phere is often caused to move locally. During the summer 
this is particularly well shown along the seashore, when the 
familiar sea and land breezes are often produced on calm 
days and nights. During the day the land becomes warm 
and air tends to flow toward the warm areas from the cool 
sea, thus producing a very refreshing sea breeze (Fig. 37). 
This breeze begins to blow 
late in the morning, and 
continues until the power 
of the sun has decidedly 
diminished. It does not 
come every day, but only 
when other atmospheric 
disturbances are not mark- 
edly developed. It causes 
a peculiar disturbance of 
the normal daily tempera- 
ture curve. In ordinary 
cases the highest tempera- 
ture of the day comes in 
the mid-afternoon ; but 
when the sea breeze com- 
mences to blow, the temperature usually falls, so that the 
highest point reached during the day may be before noon 
(Fig. 39). The sea breeze does not generally extend far 
inland ; and ordinarily at a distance of ten miles from the 
coast, it is hardly perceptible. 

At night, when the land has become cooler than the ocean, 
a very gentle breeze often blows out upon the water (Fig. 
38), rarely more than a few miles from the shore. This is 
the land breeze ; and thus it will be seen that by the daily 




Fig. 38. 
The land breeze. 



80 



PHYSICAL GEOGRAPHY. 



change in temperature there is produced a local circulation 
resembling in a small way the more extensive continental 
or monsoon circulation which depends upon seasonal changes 
in temperature. 

Where prevailing winds blow upon the coast, as is often 
the case in the trade wind belt, the intensity of these winds 
is sometimes considerably increased during the day, by the 

combination of the 
sea breeze and the 
normal wind. 
Even on the land, 
where no tendency 
to the production 
of the sea breeze 
is present, the 
change in tem- 
perature between 
day and night pro- 
duces an effect 
upon the winds. 
The heat of the 
daytime causes the 
air to rise, and freshens the winds by increasing their 
strength. Thus during a day which is calm in the morn- 
ing, strong winds may arise, and die down as the sun 
sets. 

Along the shores of large lakes, a lake breeze analogous 
to the sea breeze may arise during hot summer days. This 
is particularly noticeable along the shores of the Great Lakes 
of North America, and it is one of the reasons for the strong 
winds which prevail in such lake shore cities as Chicago. 

Mountain and Valley Breezes. — Where the topography of 
a country is very irregular, as in mountainous regions, the 




Fig. 39. 

Diagram showing normal daily curve on a hot summer 

day, and the effect produced hy the sea breeze. 



GENERAL CIRCULATION OF THE ATMOSPHERE. 



81 




Fig. 40. 
Mountain breeze. 



change in temperature between day and night often pro- 
duces a set of winds known as the mountain and valley 
breezes. During the nighttime, the air near the surface 
becomes cool by radiation, and it therefore becomes more 
dense and contracts. This dense air slides down the moun- 
tain sides into the valleys, down which it flows, often with 
sufficient velocity to cause gales. 
Just as streams gather water, and 
thus constantly increase their ve- 
locity by additions from their up- 
per branching tributaries, so these 
down-moving air currents become 
concentrated near the outlets of 
the mountain valleys. Among the 
valleys in the Rocky Mountains, 
during the calm clear days of sum- 
mer, when no other influences are 
present to disturb the tendency, these mountain breezes, or 
we might say mountain gales, are of nightly occurrence. 

The heating of the mountain sides during the day causes 
an updraft of air, which is the valley breeze. This is much 
less intense than the mountain wind, partly because the air 
is obliged to ascend against the action of gravity, and partly 
because the upflowing air is not concentrated during its 
ascent, but is rather disseminated over the mountain and 
valley sides. The day breeze becomes most intense in the 
mid-afternoon ; and the night breeze attains its maximum 
development just before sunrise. 

This type of wind is by no means confined to mountains, 
but is very well developed in plateau regions, such for 
instance as that of central New York, near Ithaca (Fig. 40). 
This plateau is dissected by numerous deep valleys which 
converge toward the broad depression occupied by Lake 



82 PHYSICAL GEOGRAPHY. 

Cayuga. At nighttime the air flows down these valleys, 
producing perceptible breezes. Concentrated in the largei 
valley occupied by the lake, the wind often develops into a 
strong breeze during the calm summer nights ; and the 
wind lasts until eight or nine o'clock in the morning. No 
breeze of the valley type has been noticed here. It is 
probable that in the other similar valleys of this region the 
same breeze is produced ; and it may be expected in almost 
any place where the land is deeply cut by valleys. 

Eclipse and Tidal Breezes. — These are practically unim- 
portant. During total eclipses of the sun, breezes have been 
noticed whose origin seems to be due to this unusual inter- 
ference with the sun's rays. Where tides rise to a great 
height, as for instance in the Bay of Fundy, local observers 
report that an increase in the wind accompanies the rising 
tide. Little is known about this type of tidal breeze, and 
it is possible that the breeze is due to other causes. 

Irregular Winds. — These winds are irregular in direction 
and intensity, and they depend upon causes which do not 
return with regularity. In these respects they are quite dis- 
tinct from the permanent planetary winds, and from the 
regularly recurring seasonal and daily winds. They there- 
fore deserve to be grouped in a separate class. Storm winds, 
the most important of this group, are considered in Chapter 
V. Their important influence in disturbing the planetary 
circulation is well shown on Plates 9, 10, and 11. 

Accidental Winds. — These winds are rare, and depend upon 
some accidental cause which starts the air in motion. Per- 
haps the most common wind of this class is the landslip or 
avalanche blast. In mountains, and more rarely in other 
places, large masses of earth and rock are sometimes pre- 
cipitated for a considerable distance down some steep slope. 
These landslides, or avalanches, displace a considerable mass 



GENERAL CIRCULATION OF THE ATMOSPHERE. 83 

of air, and form exceedingly violent local winds, which at 
a distance of a few hundred yards have in some cases been 
known to overturn trees and houses. 

During volcanic eruptions of a violent nature, vast quan- 
tities of air are started in motion ; but the effect of these 
volcanic winds is not usually important upon the surface, 
because the displaced air is high above the ground. The 
waterfall breeze is a gentle breath of air extending out from 
the base of a waterfall. 

The Nature of Winds. — The wind is a bodily movement 
of the air, but it is not necessarily a steady movement. 
Every one has noticed that the wind blows in gusts, and that 
now it is strong, and again very light. In some respects 
these pulsations are like waves ; and it has lately been 
found that even when the wind appears to be blowing 
steadily, it is really made up of a large number of gusts or 
pulsations, which can be detected only by very delicate 
instruments. Even during strong winds there may be 
momentary calms. 

Nor are the winds a perfectly horizontal movement of the 
air. As the air moves over an irregular country, it is in 
some cases deflected upward, and in other cases downward. 
Another reason for the introduction of a vertical element 
into wind movements, is the fact that upper air is sometimes 
settling, while in other cases, as a result of convection, the air 
near the surface is ascending. 

These irregularities in air movement make the wind an 
exceedingly complex series of motions, in which the pre- 
dominating direction is horizontal, but in which also there 
are a number of vertical movements. It seems very proba- 
ble that it is these vertical movements which birds make use 
of in soaring. Such birds as hawks and eagles are able to 
float about in the air, and even to rise apparently without 



84 PHYSICAL GEOGRAPHY. 

making movements with their wings. There may be an 
internal work of the wind which these birds have found, and 
made use of in their flight, sorting out those movements 
which they need, and not being retarded by those which are 
opposed to their motion. It has been suggested by Professor 
Langley that it is not impossible that these air movements 
may be employed in aerial navigation by man himself. 



REFERENCE BOOKS. 

Ferrel. — A Popular Treatise on the Winds. Wiley & Sons, New York. 
Second edition, 1890. 8vo. $4.00. (In part a republication of Kecent 
Advances in Meteorology, Report of U. S. Signal Service for 1885, Part 
II., Washington.) 

Buchan. — Report on Atmospheric Circulation, Challenger Reports, 
Physics and Chemistry, Volume II. Eyre & Spottiswoode, London, 
England, 1889. 4to. 52s. 6d. (Contains a remarkable series of charts 
relating to temperature, pressure, and atmospheric circulation.) 

See also the general books by Davis, Waldo, Greely, and others, referred to 
at the end of the other chapters. 



CHAPTER V. 

STORMS. 



Cyclonic Storms. — As used here, a storm is any con- 
dition of cloudiness accompanied by rain. On coasts that 




Fig. 41. 

Ideal diagram of a storm. Large arrow shows path of storm; small arrows, 
inhlowing winds ; circles, lines of barometric pressure ; and shaded areas, dis- 
tribution and intensity of rain. 

rise in the paths of moist winds, clouds and rain are often 
caused by the condensation of water vapor, which results 

85 



86 PHYSICAL GEOGRAPHY. 

from the rising of the air, and the consequent cooling until 
the dew-point is reached. In the same way, air that rises as 
a result of convection may reach the dew-point, thus form- 
ing clouds and rain. These kinds of rainstorms are not of 
particular importance in northern United States, and there- 
fore need not be considered in detail. 

These causes aid in the formation of the very important 
group of storms which bring the greater part of the rain 
that falls in the northern half of this country. To these the 
name cyclonic storms may be given ; and these are not of 



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1893 

Fig. 42. 



31 Sep,] 



Fall of the barometer during the passage of the center of a hurricane 
near Ithaca, New York. 

importance merely in this part of the United States, but they 
are developed on many portions of the earth's surface. We 
may divide them into two groups, the tropical cyclones, or 
hurricanes, and the temperate latitude cyclones. 

Hurricanes. — Description. In the warm Atlantic tropical 
belt north of the equator, violent storms begin and move 
toward the American coast, along which they pass in their 
course, which is then usually northeastward across the At- 
lantic. These are the typical hurricanes ; and in the North 
Pacific similar storms occur, which are there known as 
typhoons. Storms of this nature are also found in the 



STORMS. 



87 




■v, y ?h=- 



South Pacific and Indian oceans; but none occur in the 
South Atlantic, and none appear to originate on the land. 
They are typical productions of the tropics, and in these 
regions often attain extraordinary violence ; but in our 
latitude, although they are the most severe storms that we 
experience, they have lost much of their tropical violence. 

As one of these hurricanes or 
typhoons approaches a place, 
the sea is calm and glassy, and 
the air quiet and sultry. The 
pressure decreases (Fig. 42), 
and wind begins to blow with 
increasing violence, while clouds 
overspread the sky, at first as 
a thin hazy veil, which gradually 
changes to a solid mass of dark 
clouds from which rain falls. 
The wind increases to a gale and 
gradually shifts its direction, 
while at the same time the 
barometer falls. If the place 
of observation happens to be in 
the path of the center of the 
storm, as this is neared the wind 
decreases in violence and sud- 
denly changes to a calm, while 
the sky overhead becomes clear. This is known as the 
" eye of the storm." As the storm passes onward, the 
wind begins as suddenly as it ceased ; but this time it is 
from the opposite quarter, and then, in reverse order, con- 
ditions are experienced which resemble those noticed as 
the storm approached. 

These conditions indicate that the storm is a mass of 





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Fig. 43. 

Diagram of spirally inflowing winds 
of the hurricane, together with the 
path pursued by the storm. Spiral 
movements greatly exaggerated. 



PHYSICAL GEOGEAPHY. 



whirling air, toward the center of which the winds are blow- 
ing from all directions, along spiral courses, as is shown in 
the accompanying diagrams (Figs. 41, 43, and 44). The 
hurricane is therefore not unlike the desert dust whirl which 
was described in the last chapter. Air is moving toward a 
central area, where it ascends and flows away in the air 

above. This outflowing of 
the air in the upper parts of 
the storm, is shown to exist 
by the movements of the 
upper clouds, which extend 
outward as long streamers. 

Effects. — The violence of 
the winds in a hurricane is 
almost incredible, and many 
a ship that has been drawn 
into the dangerous whirl has 
not been able to escape de- 
struction. The tendency is 
for a vessel to be whirled 
around the storm center; 
and if it happens to pass 
through the center or "eye of 
the storm," the sudden change 
in the direction of the wind 
may come so quickly that the 
ship is not able to adjust its 
course in time to prevent foundering. 

When these hurricanes pass over oceanic islands, they 
often cause much devastation ; and the destruction of sev- 
eral war vessels at the Samoan Islands, in 1889, was a result 
of one of the South Pacific hurricanes. The history of the 
West Indies, and of the southeast coast of Asia, is replete 




Fig. 44. 
Diagram showing conditions of wind 
and pressure in an actual hurricane. 



STORMS. 



89 



with instances of the destruction of stout vessels that have 
been overtaken by hurricanes or typhoons. 

Accompanying the storms, there are often great ocean 
waves which sweep over low-lying coasts, sometimes com- 
pletely destroying all life and property in the areas visited. 
On September 15, 1875, three-fourths of Indianola, Texas, 




Fig. 45. 
Tracks of August hurricanes, 1888-1893. 



was destroyed, 176 lives were lost, a million dollars' worth 
of property was destroyed, and much destruction was done 
elsewhere along the coast. The same town was again devas- 
tated on August 19 and 20, 1886. On the Ganges delta, 
many hundreds of thousands of lives have been lost as a 
result of these waves. In one storm alone, that of October 



90 PHYSICAL GEOGRAPHY. 

31, 1876, 100,000 people were killed. Even along the At- 
lantic coast of the United States, where the hurricanes are 
of much less violence than in the tropics, a vast amount of 
destruction is done by them. Not only are ships destroyed, 
but the low coasts are swept by storm waves (Fig. 82), 
as has frequently been the case on the New Jersey coast 
and on the Sea Islands of the Carolina coast. 

Path. — In the North Atlantic, the hurricanes usually move 
first toward the northwest, then they curve and pass along 
the Atlantic coast of the United States until the latitude 
of Cape Hatteras is reached, when they generally turn to 
the right and pass in a northeasterly direction out into the 
Atlantic, which they often cross (Fig. 45). However, at 
times they diverge from their path and enter the United 
States, passing northward into Canada. Thus, while we 
usually experience only the western part of the hurricane, 
at times the very center moves over the Atlantic coast 
states (Fig. 42). In the North Pacific, their path is about 
the same ; but south of the equator, instead of turning to 
the right, they are guided to the left by the deflective 
influence of the earth's rotation and the prevailing westerlies. 

The size of these storms varies very greatly ; and while 
sometimes they are very large, the area covered by the 
violent portion of them is usually not more than one or two 
hundred miles in diameter. When most violent, the area 
of the hurricane is small, and this is normally the case near 
the tropics, not far from the place of origin. By the time 
they have progressed well into the temperate latitudes, 
their area is greatly increased, and they sometimes cover 
several hundred thousand square miles. At the same time 
their energy decreases, and they may even become worn out, 
so that they lose their distinctive features, particularly when 
passing over the land. 



STOBMS. 91 

Time of Occurrence. — Another notable feature connected 
with hurricanes, is the fact that they occur most commonly 
in certain months of the year. Between the years 1493 and 
1855, 355 supposed hurricanes have been recorded at the 
West Indies ; and out of these, 287 occurred in four 
months, 42 in July, 96 in August, 80 in September, and 69 
in October. In the regions south of the equator, the hurri- 
canes come most commonly in the months of the southern 
autumn and late summer, or in other words in January, 
February, March, and April. In the North Pacific, the time 
of occurrence of the typhoons is the same as that of the 
Atlantic hurricanes. The so-called " line storm " of the 
Atlantic coast, which is expected about the middle of Sep- 
tember, is in reality one of these hurricanes. 

Cause. — In the explanation of hurricanes there are several 
peculiar features which call for consideration. We must 
bear in mind that the storms are whirling areas of air, in 
which the winds move violently in a spiral direction toward 
a center, which is a place of ascending air. The whirling of 
the winds is in a uniform direction (Figs. 41, 43, and 44), in 
the northern hemisphere being toward the left hand. The 
storms begin over the ocean and are by far the most abun- 
dant in the late summer or the autumn. Their path of 
progression is first toward the northwest, and then toward 
the northeast, after having curved around with a parabolic 
curve (Fig. 45). They are found most commonly in the 
northern hemisphere and appear to be entirely absent from 
the South Atlantic. Any explanation which does not account 
for these peculiarities cannot be satisfactory. 

Since the storms are confined to the regions near the 
tropics, or occur outside of them only after having moved to 
the north or south, we naturally look to the heat of these 
regions as the cause of the storms. The warm air is ascend- 



92 PHYSICAL GEOGRAPHY. 

ing and winds are blowing toward the place of ascent. As 
a result of the directly inflowing air, a whirling cannot 
be produced; and some cause must be found which will 
originate the spiral motion of the air. A possible cause 
for this is the deflective influence of the earth's rotation ; 
but ordinarily this can produce little effect near the 
equator, because the difference in the velocity of rotation of 
different latitudes in this belt is very slight (Fig. 21). 
Upon examining the temperature charts of the world, 
we find that the heat equator is farthest from the geo- 
graphic equator in the late summer and early autumn, and 
that it migrates farthest from the equator in the northern 
hemisphere, while in the Atlantic it is never far south of the 
equator. 

When the place of maximum heat is far from the equator, 
the influence of rotation will tend to turn the winds to the 
right as they blow in toward the place where the air is 
ascending. The farther these currents are from the equator, 
the more strongly is this tendency developed; and conse- 
quently those winds that blow toward the equator, are turned 
more than those that move in from the equator. Thus a 
whirl is begun, which in the northern hemisphere, always has 
its winds turning toward the left hand. This whirl may 
best be started in the summer or late autumn. The con- 
ditions are never favorable to the production of hurricanes 
in the South Atlantic, because the heat equator does not 
migrate far into that ocean. 

The almost exclusive development of hurricanes over the 
oceans, is probably due to the presence of moisture-laden 
winds in these regions, as well as to the very uniform condi- 
tions that exist there. Water vapor is a great storehouse of 
energy, and it is estimated that the heat needed to form a 
pound of water vapor, would melt several pounds of iron. 



STOBMS. 93 

When the vapor condenses, this heat adds to the energy of 
the storm, and thus violent storms form over the ocean, where 
there is much vapor in the air ; but over the land the condi- 
tions are not so favorable. The condensation of the vapor 
aids the air in rising, and the very rising causes the con- 
densation of more vapor, so that air is drawn toward the 
center with great velocity ; and this is maintained for days, 
and possibly for over a week, by the constant supply of the 
necessary energy in the form of heat which was latent, and 
which becomes apparent when the vapor condenses. As 
the storm progresses into colder latitudes, its energy de- 
creases, and in time it dies out. 

We are able to find a satisfactory explanation of the path 
of the hurricane, in a combination of the prevailing winds 
and the earth's rotation. The storm starts in the trade-wind 
belt, but it rises above this belt into the upper air of the 
anti-trades. The one set of winds tends to blow the storm 
toward the southwest, the other toward the northeast (in the 
northern hemisphere), and the hurricane often remains nearly 
stationary for a day or two, as if in doubt which way to 
move. Eventually it begins to move in a northwest direc- 
tion toward the land, and soon it comes under the influence 
of the earth's rotation and the prevailing westerlies. This 
increases in effect as the path more nearly approaches a 
northerly direction, and the storms generally turn in the 
latitude of the region between Florida and Cape Hatteras. 

Temperate Latitude Cyclones : Resemblance to Hurricanes. — 
In many respects these storms bear a resemblance to the 
tropical cyclones ; and until quite recently it was common 
among meteorologists to consider the two classes as related 
phenomena dependent upon similar causes. These storms 
are the ones which bring the greater part of the rain to the 
northern United States, and upon which depend most of 



94 



PHYSICAL GEOGRAPHY. 



the weather changes of the northern temperate latitudes. 
The " northeast storms " of New England, so called because 
they bear damp northeast winds, belong to this class. Every 
part of the east experiences them, and their importance is 
very great. 

So close is the resemblance between hurricanes and tem- 
perate latitude cyclones, that when the latter are violent, it 




Fig. 46. 
Map showing path pursued by a storm and the conditions which accompany it. 

is quite impossible to distinguish the two kinds of storms. 
There is a resemblance in form, in winds, and in general 
behavior (compare Figs. 44 and 46). Both kinds of storms 
are great whirling masses of air, in which there are clouds 
from which rain falls ; and the storm area progresses from 
one place to another. The winds move along a spiral track 
toward a central area of low pressure, where the air is 
apparently ascending. In a part of their course, where they 



STORMS. 



95 



cross the North Atlantic, the paths of the two kinds of 
storms are practically the same (Fig. 48). 

Differences from Hurricanes. — Notwithstanding these 
resemblances, there are so many differences that we are 
warranted in considering hurricanes and temperate latitude 
cyclones as separate phenomena. One of the most striking 
differences is that of size ; for while the hurricanes usually 




Fig. 47. 

Paths of low-pressure areas, December, 1892. Large figures show the number 

of the storms, the small figures are days of the month. 

begin as small storms, they may cover a large area when 
they have passed far into the temperate latitudes ; but 
the temperate latitude cyclones may cover great areas even 
shortly after their formation. The cyclonic disturbances 
may extend over the entire eastern third of the country, 
from Canada to the Gulf, and from the Atlantic to the 
Mississippi. The hurricanes are most violent shortly after 



96 



PHYSICAL GEOGRAPHY. 



tliey are formed, while the temperate latitude cyclones often 
develop violence as they proceed on their course. While cy- 
clones may at times become very violent, they never attain the 
intensity which is noticed in some hurricanes. The whirling 
of the air in the temperate latitude cyclones is not so dis- 
tinct as in the hurricanes (Figs. 44 and 46), and, in them, 
there is rarely if ever a distinct "eye." 




Fig. 48. 

Average Sturm tracks. Relative abundance indicated by numbers showing the 

total number between the years 1878 and 1887. 

While hurricanes are most commonly developed in the 
autumn, temperate latitude cyclones occur in all seasons of 
the year, but are most numerous and violent in the winter. 
They do not develop in tropical latitudes, but are formed in 
various parts of the temperate zone. Some of them begin in 
the Pacific Ocean, others start in the southwestern part of 
this country, while others are first noticed in the northwest. 



STORMS. 



97 



Their path of progression does not show the peculiar 
curving so noticeable in the tracks of hurricanes ; but their 
direction is usually toward the east or northeast (Figs. 47 
and 49). If they begin in the Pacific or the northwest, they 
move in an easterly direction across northern United States 
or southern Canada ; and the center very commonly passes 
over the Great Lakes and down the valley of the St. Law- 




Tracks of first decade of month 

Tracks of second decade of month 

.-.-.^n-TTv^TraclcB fxom_21st to 31st, inclusive 

*•:■:-. '■.'/.^■Indicates fog "belts 
^^s^Indicates the position in wEiehneld'ioa 
or icebergs were observed 

B.D.Smmi.Tl.T. 



Fig. 49. 

Tracks of low-pressure areas (both hurricanes and temperate latitude cyclones) , 
October, 1892. Number of the storm indicated by large figures, dates by small 
figures. 

reuce. If they have their beginning in the southwest (Fig. 
47), they first move northward, then curving to the right, 
they pass out upon the Atlantic. 

The paths of the hurricanes, and nearly all of the north tem- 
perate latitude cyclones, converge toward the Nova Scotia- 
Newfoundland region, and then remain nearly parallel across 
the Atlantic. Sometimes these storms begin in the Pacific, 



98 PHYSICAL GEOGRAPHY. 

and pass across the United States, the Atlantic, and Europe, 
thus going nearly around the earth (Fig. 48). While the 
path of progression is usually regular, there are many minor 
irregularities of a peculiar and rather exceptional nature 
(Fig. 49). The origin of these is not well understood. 

Effects. — The effects of these storms in northern United 
States are very important ; and they are not confined to this 
region, but occur in Asia, Europe, and the south temperate 
latitudes. In the United States, the storms usually come 
from the west, and hence from the interior, while in Europe 
they come from the ocean. They bring to us the greater 
part of our rain and snow ; they are the main cause for 
thunderstorms and tornadoes ; they produce many of our 
most striking winds ; and they are the cause for many of 
the changes in temperature which we experience. The 
warm south winds of the winter, and the heated spells and 
droughts of the summer, as well as the cold northwest blasts 
of winter, have their origin in these cyclonic disturbances. 
At times the violence of the cyclones is so great that much 
destruction is accomplished both on the land and on the 
water. They are particularly destructive on the ocean, and 
nearly every winter the fishing fleet and coasting vessels 
suffer from their destructiveness. 

Winds. — The winds of the temperate latitude cyclones 
vary in force, as well as in direction. Some storms have 
gentle winds, while in others they are very violent ; and in dif- 
ferent parts of the same storm the velocity may vary greatly. 
On the land the)' are usually less violent than on the water, 
because the irregularities tend to destroy them by friction. 

If a storm is passing over a given place, the direction of 
the wind changes during its progress ; and the points of the 
compass through which the wind veers, depend upon the 
position of the storm center. If it is north of the place of 



STORMS. 99 

observation, the kind of change will be very different from 
that which occurs when the storm center is toward the 
south. The best way to understand these changes is to 
study the weather maps and notice the change of wind as 
the storms progress on their path. 

Certain special kinds of winds are generated in cyclonic 
disturbances. On the southern side of a storm, warm winds 
are drawn in from southern latitudes ; and in winter these 
may cause a snowstorm to change to rain. In Italy, these 
warm southern winds come from the heated desert region 
of northern Africa, and hence are usually dry. In that 
country they are known as the sirocco ; and this same type 
of wind is also developed in the United States. Here, how- 
ever, the sirocco is not dry, but is generally warm and often 
damp. In southern New England it brings damp air from 
the Atlantic Ocean ; and this air is warm because it comes 
from the area influenced by the Gulf Stream. 

A peculiar type of wind known as the foehn is developed 
in Switzerland, where air is drawn over the Alps by the 
passage of a storm center over central Europe. This air, 
drawn over the Italian side of the mountains, is caused to 
give up much of its moisture as it rises and cools. It is 
drawn down the northern side of the Alps with considerable 
velocity, and as it descends it warms dynamically. There- 
fore, the foehn is a dry and very warm wind, which in win- 
ter will often remove a layer of snow by direct evapora- 
tion. Its dryness is so remarkable that it has been thought 
to be a hot breath from the Sahara. 

A similar wind is caused by the passage of storm centers 
east of the Rocky Mountains ; and in that region it is known 
as the chinook. It is developed along the eastern base of 
the Rockies from Colorado to Montana, and its peculiarities 
are the same as those of the foehn. In the winter it often 



100 PHYSICAL GEOGRAPHY. 

causes an unseasonable rise in the temperature, and snow 
disappears before it with great rapidity. 

On the western or rear side of cyclones, instead of warm 
there are cold winds. Here the air comes from cold north- 
ern lands, and in a measure also from the upper layers of the 
air. When very violent, these cold north or northwest winds 
are known as blizzards, and they often bear with them vio- 
lent squalls of snow. The true home of the blizzard is the 
northwest ; but even in the plateau region of central New 
York, true blizzards of a somewhat milder form, often suc- 
ceed the severe winter snowstorms. In Europe, the same 
form of wind is developed; and in Texas the norther is a 
wind of similar origin. 

Anticy •clones. — Between well-developed cyclones, there are 
usually areas of high pressure, which are known as anti- 
cyclones. In these, the air is slowly settling l from upper 
parts of the atmosphere, and violent winds are not produced. 
The air is dry and clear, and hence radiation proceeds rap- 
idly, so that at night the temperatures often descend to very 
low degrees. 

While the air in these anticyclones is quiet, violent winds 
are often present at the margin, and particularly when the 
margins merge into the rear side of cyclones. Indeed, there 
seems to be a certain association between the cyclones and 
anticyclones, as if the down-settling air of the latter entered 
as a part of the whirl of the former. These conditions give 
us the cold waves of winter and the cool spells of summer 
(Figs. 63 and 64). 

Cause. — Until recently it was quite commonly believed 
that the origin of temperate latitude cyclones was the same 

1 When air settles slowly, the dynamic heating is not marked. Hence this 
settling air in anticyclones usually reaches the earth with a low temperature ; 
but there has been some warming, and the air is not so cold as when it started. 



STORMS. 101 

as that of hurricanes. In objection to this theory it may be 
said that convection does not seem capable of accounting for 
these great disturbances. In the first place, they cover an 
area often having a diameter of more than a thousand miles, 
but extend to a height of only two or three miles. Moreover, 
they are most violent and best developed in winter, when con- 
vection is least active. Recent studies seem to show that the 
cause for these storms is aloft, not at the ground. 

While it cannot be considered proven that convection is 
not the cause, there are so many reasons for doubting this 
explanation, that it certainly cannot be accepted ; and we 
are now without an explanation for these remarkable, though 
common, atmospheric disturbances. They pass across the 
country like a series of waves in the air ; and it is possible 
that the great circumpolar whirl is thus thrown into waves, 
and that these disturbances are merely a secondary part of 
this planetary circulation. Recent studies seem also to show 
that there is some relation between them and magnetism; 
but we cannot feel certain of these suggestions. 

The path followed by these cyclones and anticyclones is 
easily explained. They are borne along in the whirl of air 
which moves about the pole, and hence their direction is from 
west to east. As a result of the influence of the earth's rota- 
tion, upon the air currents the storms are carried along their 
regular paths. The winds in the storms cause a whirling in 
the same direction as that of the hurricane, and for the same 
reason, — those on the northern side are most deflected. 

Secondary Storms. — Aside from the greater general dis- 
turbances, there are certain minor phenomena of cyclonic 
storms, which attract much attention because of their vio- 
lence. The two most important of these are thunderstorms 
and tornadoes. 

Thunderstorms. — When moist air rises as a result of 



102 



PHYSICAL GEOGRAPHY. 



convection, if the ascent carries the air high enough for the 
dew-point to be reached, clouds may form and rain fall. In 
such cases electricity may be generated, and lightning and 
thunder may accompany the rain. In the belt of doldrums, 
the ascent of the moist air causes frequent thunderstorms 
during the day ; and in summer, the rising air among the 
mountains may cause the formation of thunderclouds and 
rains. In this class of storm there is no distinct whirl, but a 
simple ascent of moist air. 

In central and eastern United States, thunderstorms are 

common in sum- 
mer ; and they 
also are the result 
of uprising moist 
air. That this is 
so, is shown by 
the fact that they 
occur almost ex- 
clusively in sum- 
mer, and near the 
close of hot, sul- 
try days. On 
these days, one 
may often witness the development of such a storm, if the 
place of observation is sufficiently elevated to command a 
wide-extending view (Fig. 50). Clouds begin to develop ; 
and if they are seen from below, their bases are found to be 
flat, marking the plane at which the rising air reaches the 
dew-point. 

When seen at one side, mound-like masses of clouds, often 
of mountainous heights, are found to rise above the even 
base. If the observer is well to one side of the cloud, it will 
be noticed that as the storm develops, the form is quite like 




Fig. 50. 
Photograph of a distant thunderstorm. 



STORMS. 



103 



that of an anvil (Fig. 50). At high elevations, the clouds 
extend out in front of the storm, marking the upper outflow 
of the air. The great elevation of the cloud mass is due to 
the fact that the air continues to rise to these heights, and 
the vapor to condense as the temperature descends. 

Most of our thunderstorms are a part of moderately 
developed cyclonic disturbances, and they occur most com- 
monly in the southern part of these storms. Here warm 
moist air is being drawn 
in toward the storm 
center, and hence the 
conditions favoring the 
development of thun- 
der-storms are pro- 
duced. As the storm 
center progresses, the 
area in which thunder- 
storms may develop al- 
so moves eastward, and 
any single storm will be 
found to have the same 
path (Fig. 51). Some 
thunderstorms have 
passed entirely 




Fig. 51. 
Progression of a thunderstorm in Massachu- 
setts. The figures represent the hours at 
which the storm front reached the places 
indicated by the line. 



across 

New England, while others die out after traveling a few 
miles. Some pass over a broad path, while the width of 
others is only a few hundred yards. When the path is long, 
the storm may continue into the night ; and most night thun- 
derstorms have originated, during the preceding afternoon, 
at some point far to the west. The rate of progression is 
usually not greater than 40 or 50 miles an hour. 

In the thunderstorm, after the first violent squall, that 
usually blows out from the base of the storm, the winds 



104 



PHYSICAL GEOGRAPHY. 



are generally not violent; but there is a steady and often 
heavy downfall of rain, with accompanying thunder and 
lightning. In some cases the downpour of rain is exces- 
sive ; and among the mountains of the west, there are often 
such torrents of water that the name cloudburst is given to 
them. The name is certainly warranted, for the water falls 
in sheets, in a manner which can be appreciated only after 
having seen one. These excessive rains may be due to a 
supersaturation of the air. 

Tornadoes and Waterspouts. — These extraordinarily vio- 
lent storms are fortunately small, local, and not common in 
most of the country. Like the dust whirl of the desert, or 
like the hurricane, they are whirling bodies of air, in which 
the winds blow toward a center, where they rise (Fig. 52). 
The winds blow at such terrific rates that houses are torn 

down and the parts carried 
away (Fig. 53). The news- 
papers furnish vivid descrip- 
tions of them ; and while 
they are often exaggerated, 
almost no story concerning 
the action of tornadoes is 
too incredible for belief. 1 
In the center, where the air 
is ascending, the air pressure 
is often so low that a partial 
vacuum is produced ; and the walls of houses may then be 
blown outward by the sudden expansion of the air within. 
As the tornado approaches, it appears as a great funnel- 
shaped column of black cloud (Fig. 52), in which there are 
signs of violent commotion. As it comes nearer, a roaring 
noise is heard ; and as the cloud overspreads the sky, rain or 
1 In newspaper accounts they are usually called cyclones. 




Fig. 52. 
View of a tornado. 



STORMS. 



105 



hail falls ; but this ceases in the violent part of the tornado, 
where the air is rising so rapidly that these forms of water 
cannot descend. At first there is no wind, then suddenly a 
gale springs up, and almost' immediately its violence becomes 
so great that houses and trees are felled. On opposite sides 
of the storm the wind moves spirally toward the center. 

The tornado usually progresses at a rate of from 25 tc» 
40 miles an hour. Its width is rarely as great as a mile, 
and more often 



i.,t- 




only a few 
hundred yards, 
or even feet, 
so that it cuts 
a swathe, on 
either side of 
which no de- 
struction is ac- 
complished. 
The distance 
traversed by 
one of these 
storms is gen- 
erally not more than 30 or 40 miles, and it rarely lasts more 
than an hour. They do not occur in large numbers outside 
of the central states of the Mississippi valley, although they 
do occasionally occur in the east. West of Dakota they are 
not known. They not uncommonly occur in association 
with thunderstorms ; and like these, they come after hot, sul- 
try days, in areas covered by the southern portions of cyclonic 
storms. Their movement is almost invariably eastward. 

In part at least, tornadoes are due to convection ; and the 
reason for their abundance in the Mississippi valley seems 
to be that warm, moist air is drawn up that valley toward the 



Fig. 53. 
Effect of a tornado at Lawrence, Mass., July 26, 1890. 



106 



PHYSICAL GEOGRAPHY. 



storm center, while above it there is a colder layer of eastward 
moving air. Therefore the conditions of the atmosphere 
are peculiarly unstable ; and the increased heat caused by the 
sun, starts an overturning which soon takes the form of a 
violent whirl. This is not possible in the far west, where 

the lower air is dry; 
and in the east, the 
atmosphere is rarely in 
a sufficiently unstable 
state for this violent 
overturning. 

When the tornado 
develops or passes over 
the sea, or over a large 
lake, it takes the form 
of a waterspout. It 
is doubtful if these 
waterspouts are col- 
umns of water, as is often stated ; but there is probably a 
conical wave in the center. 




Fig. 54. 
Distribution of tornadoes 1794-1881, the in- 
tensity of shading showing greatest abun- 
dance. Darkest more than 35, medium shade 
25-35, lightest shade less than 25. 



REFERENCE BOOKS. 

Ferrel. — A Popular Treatise on the Winds. (For price, etc., see refer- 
ence at end of Chapter IV.) 

Davis. — Whirlwinds, Cyclones, and Tornadoes. Lee & Shepard, Boston, 
1884. 24mo. $0.50. (Reprinted from " Science.") 

Finley. — Report on the Characters of Six Hundred Tornadoes. 
Professional Papers No. 7, U. S. Signal Service, Washington, 1884. 

Finley. — Tornadoes. Hine, New York, 1887. 12mo. $1.00. (Based 
mainly upon previous publications in the U. S. Signal Service Reports, etc.) 

The Monthly Weather Review and the Daily Weather Maps, published 
by the Weather Bureau at Washington, and the Pilot Charts, published 
by the Hydrographic Office of the Navy Department at Washington, will 
be found invaluable in laboratory instruction. Teachers who are inter- 
ested can probably obtain these upon application. 



CHAPTER VI. 

THE MOISTURE OF THE ATMOSPHERE. 

Dew. — When the temperature of the air descends far 
enough, a point is reached when there must be a condensa- 
tion of some of the contained moisture, because the ability 
of the air to carry water vapor, depends in large measure 
upon the temperature. With dry air, the temperature must 
be lowered much farther than with damp or humid air ; and 
on the sultry days of summer, a pitcher of ice water lowers 
the temperature of the air in contact with it sufficiently to 
cause the condensation of some of the vapor on the outside 
of the pitcher, which is said to "sweat." 

When the ground becomes cold at night, the lower air is 
also cooled, and that which is in contact with the ground 
may give up some of its vapor as dew. The temperature at 
which this will happen, naturally depends upon the amount 
of vapor in the air ; and in the tropics, where the hot air is 
very humid, the amount of dew that forms at night is often 
very great. Even the coolness of the late afternoon is often 
sufficient to cause the condensation of dew in the tropics ; 
and during our own summer days, one often notices that the 
grass is wet with dew even before dark. 

Dew forms most readily on those bodies that cool by radia- 
tion most quickly. Thus grass and leaves are dew-covered 
sooner than soil. During some nights, even when the air is 
quite humid, dew is not formed. By interfering with radia- 
tion, clouds tend to prevent the formation of dew ; and as a 

107 



108 PHYSICAL GEOGRAPHY. 

result of the stirring of the air, and the inflow of new sup 
plies of air, wind tends to check dew formation. Because 
the air is more humid, dew is formed more readily near 
streams or swamps than in dry places. Dew is heavier in 
valleys than on hills, partly because of the greater damp- 
ness of the valleys, partly because cold air slides down into 
them from the hillsides, and partly because the air in valleys 
is more quiet than that on the hilltops. 

While the main cause for dew seems to be condensation 
of vapor from the air, recent studies show that this is not 
the only cause. At all times plants are furnishing moisture 
to the air by transpiration. Ordinarily this is evaporated; 
but at night this evaporation is checked, when the air is 
cooled, and its power for evaporation reduced because it is 
either saturated or has a high relative humidity. Then the 
moisture forms drops of water on the leaves. Thus dew is 
a result of the combination of two processes, in both of 
which the cooling of the air by contact with the earth is 
the important cause. 

Frost. — When the temperature of the dew-point is below 
freezing, the condensation of vapor takes the form of frost. 
It is not frozen dew, but vapor that has become condensed 
as a solid, instead of a liquid. In cause, and in occurrence, 
frost may be described in the same terms as those used in 
the description of dew. However, the effect of frost is 
quite different, for it causes vegetation to suffer, while dew 
refreshes vegetation. Frost is not likely to occur on windy 
or on cloudy nights, and it comes earlier in damp valleys 
than on dry hilltops. This is why the autumn foliage 
first assumes its brilliant tints in the swamps. A cover- 
ing, such as a sheet, by interfering with radiation, will 
prevent a light frost ; and in this way delicate plants may 
be protected when there is danger of frost. 



THE MOISTURE OF THE ATMOSPHERE. 109 

Fog. — This is merely the condensation of water vapor 
into the form of very tiny drops, which are so light that 
they do not readily fall to the ground. When we breathe 
the warm moist breath into the cold air of a winter day, we 
produce a tiny fog. Many of the great ocean fogs owe their 
origin to a similar cause. On the banks of Newfoundland, 
where the warm Gulf Stream is side by side with the cold 
Labrador current, fogs are produced when the winds of the 
one region pass over the other. Very extensive fogs are 
thus caused, and this has made that part of the Atlantic 
famous. When warm air is drawn northward toward storm 
centers, fogs are particularly liable to occur here. These 
and other ocean fogs often extend upon the land, as for in- 
stance on the coasts of Maine and Nova Scotia. 

A fog sometimes surrounds an iceberg, because the air 
around it is chilled. Over the surface of lakes we some- 
times see fogs developed by the chilling effects of air cur- 
rents. At times the cool water produces a fog by contact 
with warm air. In damp valleys, a valley fog is often 
formed when the air is chilled and the vapor condensed 
into particles. This is particularly liable to happen during 
nights when the conditions favoring a heavy dew are pres- 
ent. Every one must have noticed the cool dampness of 
valleys, which is so noticeable in passing along a hilly road 
just after nightfall. This often increases until some of the 
dampness forms into fog particles. One often sees valley 
fog among the mountains (Fig. 55~), and many clouds are 
nothing but fogs. As one looks down upon a valley fog, 
there is a white rolling surface, above which may rise the 
tree tops or church steeples, while everything else is hidden. 
The appearance is not unlike that which one sees in the 
mountains when above the clouds. 

By furnishing a nucleus about which the vapor may con- 



110 PHYSICAL GEOGRAPHY. 

dense, " dust " particles are important in the formation of 
some fogs. It is believed that the fogginess of London in 
part depends upon the large amount of dust in the air. 

Haze. — At times, and particularly during dry weather, 
a thin veil of blue haze extends through the atmosphere and 
partly obscures the distant landscape. Often it is so indis- 
tinct that one notices it only when an unobstructed view 




Fig. 55. 
Valley fog in the Himalayas. Mount Everest in the background. 

of far-distant hills is obtained; but during some days it 
becomes so thick and dense, that points near at hand are 
almost completely obscured, and even the sun loses its in- 
tensity, while the sky becomes dull. Haze is not damp like 
fog, and there is reason to question whether it is often due 
to water particles. Probably the greater part of the haze 
results from dust in the air ; and during droughts the air 
is often very hazy, because at such times rains have not 



THE MOISTURE OF THE ATMOSPHERE. Ill 

occurred to clear the sky, and the air is often supplied with 
much dust from forest fires. 

Mist. — At times the air is filled with minute particles 
of water, which are larger than those in a fog, and which 
therefore cause greater dampness. The mist is intermediate 
between fog and rain, and possibly it is made of numerous 
fog particles which have united. 

Clouds. — Clouds are composed of particles of moisture 
due to the condensation of water vapor. Sometimes these 
particles are very small, like those in fog, at other times they 
are made of mist, or even of raindrops, and in many cases 
of ice particles or snow crystals. They are formed when- 




Fig. 56. 
The banner cloud, caused by a moist wind blowing against a mountain peak. 

ever vapor-laden air has its temperature lowered to the dew- 
point ; and this may be caused in several ways. 

When damp air encounters a cold mountain top, clouds are 
formed, and these may surround the mountain peak or ex- 
tend beyond it like a banner (Fig. 56). Where high mountains 
extend upward in the path of the trade winds, these banner 
clouds are often produced. Air that is caused to ascend, fre- 
quently has its temperature lowered below the dew-point; 
and when this point is reached, clouds are formed. This 
may happen when air rises by convection, or when it ascends 
land elevations. During the summer, and in mountains, 
such clouds are commonly formed. The mixture of air of 



112 



PHYSICAL GEOGRAPHY. 



various temperatures often causes cloud formation, and 
this appears to be the origin of many of the clouds of the 
upper atmosphere. In reality, fog is a form of cloud ; and 




Fig. 57. 

Photographs of five common cloud forms. 

Cirrus. 



Nimbus. 
Cirro-cumulus. 



Cumulus. 
Cumulo-stratus. 



during storms, when the clouds are low, we may find our- 
selves enveloped in a true cloud mist. 

The forms of clouds are very beautiful and varied, and 
the various kinds are known under different names. The 



THE MOISTURE OF THE ATMOSPHERE. 113 

following is a classification of clouds based partly upon their 
form and partly upon their elevation : — 

Cirrus. Cumulo-stratus. 

Cirro-stratus. Nimbus. 

Cirro-cumulus. Stratus. 
Cumulus. 

The cirrus cloud (Fig. 57) is the highest form known, its 
elevation often being greater than five miles. It is so high 
that the condensation of water vapor forms ice spicules, and 
this is the reason why these clouds appear thin, white, hazy, 
and almost transparent. They drift along at very rapid rates, 
and in northern latitudes usually move toward the east, being 
carried along in the circumpolar whirl. Their form is 
variable and often remarkable. They are commonly pro- 
duced by the upper outflow of air in a cyclonic storm. 

At times the cirrus clouds occur in the form of distinct 
bands, and they are then known under the name of cirro- 
stratus. This form of cloud may completely overspread the 
sky, but its transparency is so great that the sun is visible 
through it, and during such conditions of cloudiness halos 
and coronas are commonly formed. Many varied forms of 
cirrus clouds are recognized, and various names are given to 
them. Sometimes they are frayed and torn as if by violent 
air currents. At other times they occur in bunches, arranged 
often in lines, as if produced by waves of the air, the groups 
of clouds resembling a choppy sea. When these bunches of 
upper air clouds are quite distinct, they are known as cirro- 
cumulus (Fig. 57). Oftentimes the sky is speckled with 
these clouds, and then sailors call it the mackerel sky. 

Among the most beautiful of clouds are those known as 
cumulus (Fig. 57). They are produced at a lower elevation 
than the cirrus, and are often composed of fog particles in- 



114 PHYSICAL GEOGRAPHY. 

stead of ice. When best developed, as is the case in summer, 
they are the typical thunder heads, which rise from a flat 
base, at an elevation of about a mile, and extend into the air, 
often to a height of several thousand feet above this. 
They consist of a mass of rounded, dome-like clouds, which 
often produce a very fantastic and beautiful effect, partic- 
ularly when lighted by the rays of the setting sun. 

These clouds are common, every-day occurrences in the 
belt of calms, and in summer they are often produced 
around mountain peaks, and over the heated lowlands. In 
these cases their cause is the ascension of warm moist- air ; 
and during hot summer days they may often be seen to 
form. Over the land, they are much more readily formed 
than over the water, and the presence of land is often indi- 
cated by their occurrence. When sailing along the coast of 
Florida in summer, the position of the land is often shown 
by a line of these clouds. At nighttime, when convection 
ceases, the clouds melt away and the sky clears. 

Clouds that resemble the cumulus, but differ from them in 
being more massive and banded, are known under the name 
of cumulo-stratus (Fig. 57). When this form of cloud is very 
massive, so that large parts of the sky are covered, the name 
stratus is applied. These often entirely overspread the sky, 
forming a gray, illy-defined cloud mass. Their elevation 
is usually between 600 and 3000 feet, but at times they are 
so low that they touch the earth. This is the kind of cloud 
that occurs during cyclonic storms, and then they may cover 
the sky over an area of thousands of square miles. When 
rain is falling from a cloud, it is known as nimbus (Fig. 57). 

Rain. — There is every gradation between dew and rain, 
and raindrops are often made by the union of numerous fog 
particles. The exact means by which these particles are 
gathered together, cannot be stated ; but perhaps in many 



THE MOISTURE OF THE ATMOSPHERE. 



115 



cases it is the result of the contact of particles driven against 
one another by wind, or as a result of their descent through 
the air. 

There is a very definite relation between clouds and rain, 
and the causes which produce the one form the other. The 
most important of the causes are the mixture of currents 
of different temperature, the uprising of air, and the con- 
tact of warm moist air with cold land surfaces. The 
greater part of 
the rain of the 
world falls 
either (1) from 
cumulus clouds, 
or (2) from cy- 
clonic storms, 
or (3) where 
moist winds 
blow from the 
water upon the 
land. Away from places where these conditions occur, the 
rainfall is usually light. Sometimes, though rarely, rain- 
drops fall from a clear sky. 

Snow. — This is the crystallized form assumed when water 
vapor condenses at temperatures below the freezing-point ; 
and the forms thus produced are often very beautiful and 
fantastic (Fig. 58). There is an intimate relation between 
snow and rain, and the same storm may produce snow on the 
highlands and rain on the lowlands. Many of our winter 
rainstorms are due to the fact that the snow crystals have 
been melted in their downward passage ; and the damp 
snows are a partial step in this direction (Fig. 59). The 
difference of a few degrees thus produces a very marked 
change. In the one case rain falls and speedily flows away, 




Fig. 58. 
Photographs of actual snowrlakes. 



116 



PHYSICAL GEOGRAPHY. 



while in the other case a cold covering of solid snow is laid 
upon the land, perhaps to stay for months. The clouds of 
the upper air are mostly made of ice or snow, and mountain 
peaks that extend into these upper layers, rarely receive any 
other form of precipitation. 

Hail. — At times, particularly in summer, balls of ice 
known as hailstones fall from the clouds, especially from 



*-•# Q »* 





Fig. 59. 

Photograph taken after a fall of damp snow, showing how it 
clings to vegetation. 



those accompanying thunderstorms and tornadoes. They 
are usually oval or rounded in form, and are often made 
of successive shells of clear and clouded ice. The mode of 
formation is not known ; but there is some reason for believ- 
ing that they are formed in violently moving and rising air 
currents, and that this is the reason why they so commonly 
fall on the margins of rather violent storms. 







EXPLANATION 
I OVER 200 

i ' 1 '''""■ 130-200 



n 



60-130 
20-60 

LESS THAN 20 



Face page 117. 



Raini «l 




GBAMATIC MAP 

SHOWING 

L Or THE WORLD 

^TIMETERS PER YEAR 



B.D.Seruoas.H.Y. 



d. 



THE MOISTURE OF THE ATMOSPHERE. 117 

Distribution of Rainfall in the World. — As used here, the 
term rainfall includes both rain and snow. In general there 
is a difference in the amount of rainfall according to latitude 
and altitude. Since in high latitudes and high altitudes the 
temperature of the air is low, and therefore contains little 
vapor, the amount of rain that can be condensed in these 
places is less than in the warm tropics, where the air is 
humid. Still, there is much variation in this respect, as will 
readily be seen by a glance at Plate 12. 

Without entering into the subject in great detail, a few 
notable facts shown on this chart may be pointed out. It 
will be noticed that in the belts where the trade winds blow 
upon the land, the rainfall is heavy, while in those places 
where they blow over the land, the rainfall is slight. Thus, 
as a result of this, the dry desert of the Sahara exists in the 
same latitude with several very rainy districts. 

Where the winds blow against steeply rising mountains, 
such as the Himalayas, the precipitation is very heavy. 
Even outside of the trade-wind belt, when the winds blow 
from the warm ocean upon the land, the amount of rainfall 
is often very great. If mountains intercept these winds, 
they are drained of their moisture, and pass to the opposite 
side as dry winds, producing deserts. Thus there are two 
important causes for deserts. 

In the belt of calms, where the air is almost constantly 
rising during the day, the precipitation is quite uniformly 
heavy ; and as these belts migrate, the rainy conditions are 
carried with them. Thus we may have one very wet season, 
and an opposite dry season, when the calms are replaced by 
the trades. This is the case on both sides cf the equator 
in Africa and South America. 

Usually the rainfall is heavy near the coast ; but where this 
is not the case, the winds are blowing from the land to the 



118 PHYSICAL GEOGRAPHY. 

sea. With the seasonal change in the wind direction, some 
coasts have a dry and a wet season. In the interior of 
continents, a condition of relative dryness usually prevails. 
This is not always a true desert condition, but often one of 
semi-aridity, in which the rainfall is not sufficient for suc- 
cessful agriculture. There may be every gradation be- 
tween the humid country and a desert, passing through the 
stages of semi-aridity and the climate in which droughts 
are common. 

The greatest irregularities of rainfall are noticed in 
temperate latitudes ; and these depend in part upon the 
winds, the topography, the neighborhood to the sea, and 
the occurrence of cyclonic storms. In parts of the area, 
nearly the entire rainfall comes in association with these 
storms. Bearing in mind the previous discussion of tem- 
perature, winds, and storms, the student will be able to 
understand most of the irregularities in rainfall distribution 
indicated on the accompanying charts. 

Distribution of Rainfall in the United States. — In this 
country (Plate 13), most of the features noticed on the rain- 
fall charts of the world are well illustrated ; but we have not 
the tropical conditions. On the Texas coast, the inblowing 
trades of the summer cause a heavy rainfall ; and in Florida, 
much of the rain depends upon the neighborhood of the warm 
ocean waters. The rainfall of the eastern coast is less than 
that of the western, because in the former the winds are 
mostly from the land. Still, because this region is frequently 
visited by cyclonic disturbances, there are no deserts pro- 
duced in the east. 

From Florida to Maine, the rainfall decreases quite uni- 
formly, as it should in passing from warm to cooler regions. 
On the western coast, the reverse is true, and the most humid 
part is in the north, while the southern portion is quite 



120 



PHYSICAL GEOGRAPHY. 



arid. This is due to the fact that in the northern part, the 
winds blow from the ocean against the mountains. 

Because of this, the rainfall also decreases very rapidly 
from the immediate coast toward the interior. Beyond the 
mountains of the coast, the country is either arid or in a 
truly desert condition ; and this extends even to the plateau 
states east of the Rocky Mountains. Throughout the greater 




Fig. 60. 
Rate of evaporation in the United States. Based upon observations for a year, 

in 1887-1888. 

part of the western half of the country, the rainfall is very 
slight, because there are no great water bodies to supply the 
winds with moisture. Even in the states just west of the 
Mississippi valley, the rainfall is light and quite irregular, 
because the winds are dry. Here evaporation is rapid, and 
in some parts, where the total annual rainfall is less than 
10 inches, it amounts to 100 inches (Fig. 60). 



THE MOISTURE OF THE ATMOSPHERE. 



121 



Distribution of Snowfall. — Over a very large part of the 
earth' s surface snow is impossible, and a considerable part of 
the human race has never seen it. In the United States, 
snow falls nearly everywhere except in Florida and south- 
ern Texas and California ; but it is only in high tem- 
perate and Arctic latitudes that much snow can fall upon 









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Fig. 61. 

Monthly rainfall in the West, showing the heavy winter rains of Washington, 
in contrast with the normal condition of heaviest rainfall in summer. 
Also showing differences in amount in inches of rain. 

the lowlands. Even under the equator it may fall on high 
mountain peaks. There is much variation in the distribution 
of snow, both from season to season, and from place to place. 
Where the temperatures are low, the snow remains upon 
the ground during the winter ; but in many places it stays 
only for a short time. In high mountains, where the snow- 
fall is great, and where there is very little melting, it may 



122 



PHYSICAL GEOGRAPHY. 



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produce glaciers as a result of the 
accumulation of many winters' snow- 
fall. The same is true in parts of 
the Arctic and Antarctic lands, and 
in these cold places, even the summer 
precipitation is mostly in the form of 
snow. 

Seasonal Distribution of Rainfall. 
— Many parts of the earth have dry 
and wet seasons ; and as has already 
been explained, this is usually due 
to a change in wind. In equatorial 
Africa, among the headwaters of the 
Nile, the migration of the belt of 
calms causes such a condition, and 
the same is true of the llanos of 
Venezuela and the campos of Brazil. 
The blowing of the monsoons upon 
the coast of Asia, and elsewhere, 
causes very rainy conditions which 
are quite absent when the monsoon 
winds blow from the land. At Cher- 
rap unji, where the rainfall is as great 
as 500 inches a year, the amount fall- 
ing in December is only 0.2 inches, 
while in July it is over 130 inches. 
This excessive rainfall, which is the 
greatest on the earth, is caused by 
the blowing of the monsoons against 
a steeply rising mountain face. On 
the western coast of the United 
States, particularly in Washington 
and Oregon, the winter rainfall is 



THE MOISTURE OF THE ATMOSPHERE. 123 

heavy, while in summer it is light (Fig. 61). This is due 
to the damp winter winds from the Pacific. 

In the central and eastern states, the distribution of rain- 
fall is very irregular, and it depends upon the nature and 
frequency of cyclonic storms. Some seasons are very dry, 
and then droughts may occur ; but there is no regularity in 
the recurrence of these periods. Fig. 62 illustrates this 
variation in the western states. 

Irregularities of Rainfall. — The normal rain is a steady 
and rather quiet downpour ; but at times, particularly in 
connection with thunderstorms, the rainfall may be very 
heavy, and then more rain may fall in a few minutes than 
during an ordinary cyclonic storm lasting for a day or two. 
For instance, at Syracuse, New York, 8 inches of rain fell 
in one day, June 8, 1876 ; and in June, 1886, over 21 inches 
fell in 24 hours at Alexandria, Louisiana. The effect of 
such a sudden deluge of water in swelling the streams and 
wearing away the land is very important. The cloud-bursts 
of the Rocky Mountains furnish other instances of very 
remarkable rainfalls occurring in a short period of time. 
Where the rains are excessive in violence, the aoil is some- 
times washed away from steep slopes, leaving the bare rock 
exposed to the air. This is the case in that region of 
remarkable rainfall in India. 



REFERENCE BOOKS. 

Tyndall. — The Forms of Water (International Scientific Series). Appleton 

& Co., New York, 1872. 12mo. $1.50. 
Schott. — Precipitation, etc., of the United States. Second edition, 

1885, Smithsonian Contributions to Knowledge, Vol. XXIV., 1885. 

Smithsonian Institution, Washington, D.C. $6.00. 
Harrington. — Rainfall and Snow of the United States. Bulletin C, 

Weather Bureau, Washington, 1894. (Many valuable charts.) 



CHAPTER VII. 

WEATHER AND CLIMATE. 

Weather. — Climate is the sum and average of weather, 
which includes the daily change in temperature, pressure, 
wind, rain, etc. The climate shows the general condition, 
while weather deals with the special instances of changes 
in the atmosphere. The data obtained in a study of the 
weather furnish the basis for a knowledge of the climate, 
and thus the two subjects grade into one another. Already, 
in the previous pages, much has been said concerning weather 
and climate -, 1 but now a few statements upon the subject are 
made as a kind of summary. 

Tropical and Arctic. — There is much difference in the 
variety of weather in various parts of the earth. Over the 
ocean, the weather conditions are less variable than on 
the land, and the greatest variation is found in temperate 
latitudes. Day after day, the weather in the belt of calms is 
nearly the same, the clear nights being followed by cloudy 
days with frequent rains, and the temperature being high 
and not very variable. In the belt of trade winds, the air 
moves rather steadily toward the equator, and the tempera- 
ture is high. When these winds blow over the land, their 
dryness produces desert conditions ; and when they blow 
upon the land, heavy rains are caused. Thunderstorms 

1 Many of the foregoing figures and plates illustrate this chapter as well. 

124 



WEATHER AND CLIMATE. 125 

may occur, and now and then a hurricane may develop, 
bringing with it violent winds and heavy rains. 

In the polar regions the winter season is marked by 
uniform cold, and the storms always bring snow. During 
the summer there is no marked day and night alternation 
in temperature ; and although the air is warmer than in 
winter, the temperature is uniformly low and snowstorms 
may occur. 

Temperate Latitude Weather. — Taking the United States 
as typical of the temperate latitudes, we will examine the 
weather conditions of several sections. On the Pacific coast, 
north of central California, the days of summer are dry and 
warm, and the nights become quite cool. In the winter the 
warm winds from the Pacific blow upon the land, producing 
frequent rains during the day ; but the temperature of the 
day, and even of the night, is moderate. 

In the high mountains east of this, the air is cold, and 
even the summer storms often produce snow instead of rain. 
The temperature of day and night is low. In the desert 
regions between the mountains, storms rarely occur, and the 
air is quite constantly clear and dry. Occasionally, espe- 
cially in summer, there are heavy thunderstorms, particu- 
larly among the mountains ; but in some of the deserts, as 
for instance that of Arizona, there is almost no rainfall 
(Plate 13). During the summer day, the ground and air 
become highly heated, and at night low temperatures are 
produced by radiation. 

On the plains of Dakota, Montana, Manitoba, etc., the 
air is prevailingly dry ; and during the summer, the tern 
perature of the day becomes high, while the nights are 
cool. During the winter, excessively cold spells are liable 
to occur, and temperatures as low as —30° are not uncom- 
mon. This region is subjected to the influence of cyclones 



126 PHYSICAL GEOGRAPHY. 

and anticyclones, with their accompanying conditions of 
rain or clear weather and variable winds. During the 
winter, there may be very heavy snowstorms, and at times 
extremely violent blizzards ; and, following these, the warm 
chinook wind may cause an unseasonable rise in temper- 
ature. Farther south similar conditions prevail, but the 
weather changes are less intense. On the dry plains of 
Texas, the temperature ranges are extreme ; but neither the 
chinook nor the blizzard occurs, though a cold norther some- 
times produces a very severe weather change. 

Along the coast of the southern states, high temperatures 
are experienced, and the ranges from season to season, and 
from day to night, are not great. Rainstorms are produced 
by the blowing of the winds from the warm ocean upon the 
land ; and in the autumn, violent tropical hurricanes often 
visit the coast. No snow falls, but during the winter, when 
a cold wave spreads over the country, freezing temperatures 
may at times extend into this belt. 

In the more northern states of the Mississippi valley, 
the weather of the winter is cold, snowstorms accompany 
cyclonic disturbances, and extremely low temperatures are 
produced during anti-cyclonic conditions. There are great 
daily temperature ranges, as well as some of an irregular na- 
ture. During the summer, cyclonic storms are less common, 
and they come at irregular intervals, and there may be long 
periods of drought. These droughts occur when the cyclonic 
disturbances are of moderate intensity, and the storm centers 
far to the north, in the Canadian territory. During these 
conditions, warm air is drawn in from the south toward 
the storm center, and this raises the temperature but does 
not produce rain. Under favorable conditions, thunder- 
storms and tornadoes may arise during the passage of low- 
pressure areas. 



WEATHER AND CLIMATE. 



127 



In southern Canada, New York, and New England, the 
weather is very variable and irregular. In the winter, snow- 
storms occur, and these are sometimes very heavy, particu- 
larly in the northern part of the area. Over a large part of 
the region, storms are of sufficient frequency, and the cold 
sufficiently intense and uniform, to allow the snows to ac- 
cumulate during the winter and remain upon the ground 




Fig. 63. 

Conditions of wind, pressure and temperature accompanying a cold wave, 
March 14, 1895. 



until spring. In the southern part of the district the snow- 
storms may change to rain, or they may be followed by warm 
weather, causing the winter thaws, as a result of the 
inblowing wind from the south, which is drawn toward the 
storm center. Along the coast, cold east winds are often 
drawn from the ocean, particularly during storms. The 
cold waves which often follow the storms, cover the land 
with a blanket of very cold air (Figs. 63 and 64), through 



128 



PHYSICAL GEOGRAPHY. 



which ladiabion proceeds with ease, giving us our coldest 
winter weather ; but the cold is not so intense as in the dry 
interior area of the northwest. 

In the summer, storms are less frequent and less violent ; 
but still they produce an effect upon the weather. When 
they are not intense, the warm air drawn in from the south, 
produces days of excessive heat and sultriness, during which 
thunderstorms may come ; or a continuation of this condi- 
tion may cause summer droughts. Along the seacoast, fogs 
are sometimes blown in upon the land, or the cool sea breeze 
may temper the heat of the summer day (Fig. 39). Well- 
developed cyclonic storms may arise : and in the autumn 



50° 
40° 


Tues.Mar.12 

NOON 


March 13 


March 14 

NOON 


March 15 

NOON 


March 16 

NOON 


March 17 

NOON 


50 c 

30- 
20 : 
10' 
0° 


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1895 









Fig. 64. 
Sudden descent in temperature during passage of a cold wave at Ithaca, N. Y. 



these become more frequent, and the region may then be 
visited by one of the West Indian hurricanes. During both 
summer and winter, the winds are very variable in force and 
direction. 

This, which may be considered the typical weather of the 
temperate latitude, has for its main feature extreme irreg- 
ularhVy and variability. From place to place there is much 
variation; and even at the same place, the weather of suc- 
cessive years is quite different. These are essentially the 
conditions that prevail in Europe ; but here the winds are 
damper because their prevailing direction is from the west, 
and hence from over the Atlantic. Since there is less land, 



WEATHER AND CLIMATE. 



129 



there is much less variability in the weather conditions of 
the temperate latitudes of the southern hemisphere. 

Climate. — The earth may be divided into climatic belts, 
the three primary zones being based upon difference in lati- 
tude, and hence in supply of solar energy. These zones are 







Fig. 65. 
Climatic zones. 

the tropical, temperate, and arctic (Fig. 65}. Speaking 
generally, the tropical climate is characterized by high tem- 
peratures throughout the year, the arctic by low tempera- 
tures in all seasons, and the temperate by variability, and a 
marked change in the two opposite seasons of summer and 
winter. There are many exceptions to this general state- 



130 PHYSICAL GEOGRAPHY. 

ment, and each zone must be subdivided into oceanic, insu- 
lar, interior, and upland climates. 

Tropical Climate. — Between the tropics, the climate of 
the oceans and coasts is mostly warm and equable. The 
rainfall is considerable, though to this there are numerous 
exceptions, as for instance on those coasts from which the 
trade winds blow toward the sea. The doldrum belt is one 
of excessive rain and very uniform conditions of tempera- 
ture ; but that of the trade winds has a more variable 
climate. In the interior of the continents, there is much 
variation, though the uniform condition is that of high tem- 
perature. The temperature ranges are greater than on the 
ocean, and the average temperature is also higher. 

There is every gradation between the regions of heavy 
equatorial rains, and deserts. In the belt of calms the rains 
are heavy, while in the trade- wind belts, the dry, south- 
moving winds often produce a truly desert condition. Thus 
the desert of Sahara, with a rainfall of less than 20 inches, 
and in some places with almost no rainfall, is on the same 
latitude with the region of eastern Central America, where 
the rainfall is over 200 inches. In the narrow zone which 
is alternately occupied by the belt of calms and the trade 
winds, the climate of one season is dry, and that of the 
other is very damp. 

As a result of the monsoon condition, a peculiar climate 
is produced in India. There are three seasons : one cool and 
dry, when the winds blow from the interior ; the second hot 
and dry, when the sun's heat becomes intense ; and the third 
a wet season, when the monsoons blow upon the land. 

Temperate Climate. — For the most part, the climate of the 
north temperate zone is very variable, and the year is divided 
into two seasons of extremes, — the summer and the winter, 
— with intermediate seasons of spring and autumn, which are 



WEATHEB AND CLIMATE. 131 

gradations between the summer and winter. However, this 
belt is divisible into several minor zones, of which we may 
consider five : the west coast, the east coast, the interior, the 
mountain, and the inter-montane zones. 

The climate of the west coasts is comparatively equable 
and damp, because of the influence of the ocean, from which 
the winds blow as a part of the circumpolar whirl. On the 
eastern coast, the climate is largely influenced by the condi- 
tions of the interior, because the wind comes from this direc- 
tion ; but the neighborhood of the ocean somewhat modifies 
the climate, and it is not so extreme as that of the interior. 

As has been said, the interior climate is very extreme ; and 
the cyclones and anticyclones which affect nearly the entire 
temperate belt, are much more marked than in other parts of 
the zone. The climate cf the mountains resembles that of the 
region in which they are situated ; but it is colder and usually 
more humid. Thus frost-covered mountains may rise from 
desert plateaus. Among the high mountains, much of the 
precipitation is in the form of snow. Between the moun- 
tains, and on the leeward side of them, arid and even desert 
conditions result from the fact that the winds are drained of 
their moisture in passing over the mountains. 

This great variability in condition is strikingly shown in 
passing around the earth on one of the parallels of latitude, 
such, for instance, as that of 50° N., as described by Davis 
in his " Elementary Meteorology." Passing from the equable 
climate of the Atlantic, it crosses the European continent 
through regions so temperate that they are densely popu- 
lated. In Asia, great plateau deserts are encountered, and 
on the Pacific coast the climate is quite severe ; but in that 
ocean very equable conditions are found. The parallel enters 
British Columbia, where the climate is moderate and moist, 
passes over the high snow-covered mountains, and crosses 



132 PHYSICAL GEOGRAPHY. 

the great interior region of extreme cold, north of the Great 
Lakes, emerging across the Labrador peninsula to the Atlan- 
tic, in the middle of which the climate is modified by the 
warm Gulf Stream. 

Arctic Climate. — The arctic climate is one of extreme 
and prolonged cold, and the ground is covered with snow 
for the greater part of the year. During the winter, the sun 
remains below the horizon, and in summer it does not set. 
On high mountains which rise into the cold layers of the 
upper air, 1 many of the conditions of the arctic climate 
extend into the temperate, and even into the tropical 
zones (Fig. 65) . Between the tropics, a temperate climate is 
found at moderate elevations on the mountain sides. 

Minor Variations. — Aside from the larger divisions of 
climate, there are many smaller ones. The climate may 
change very perceptibly even in a short distance, „as, for 
instance, in going from the southern sunny aide of a 
mountain to the shaded northern side (Fig. 68). Even 
a lake of moderate size may produce a perceptible influ- 
ence upon climate ; and it is not uncommon to find a belt 
adapted to fruit-raising whose boundaries include but a 
small area. 

Changes in Climate. — There is an abundance of geological 
evidence to prove that there have been great changes in 
climate in parts of the earth's surface ; and there is some 
reason for believing that there have been changes in climate 
within historical times. Recent studies in Europe seem to 
show that there is a period of slight variation in climate, 
extending over thirty-six years. The change is from drier 
and warmer, to cooler and moister conditions ; and we are at 
present in the midst of the warm, dry part of the cycle. 

1 Of course, this applies only to the cold ; for the position of the sun does 
not resemble that of the Arctic. 



WEATHER AND CLIMATE. 133 

This is merely a suggestion, and cannot be accepted as a 
definitely established fact. 

Of the geological evidence we are more certain. During 
the earlier ages of the earth's history, the climate of the 
globe seems to have been more moderate and uniform. 
Fossils of animals and plants that are at present confined to 
warm latitudes, are found preserved in rocks which are buried 
beneath perpetual snow. When these forms of life existed, 
this part of the earth must have been much warmer than now. 

On the other hand, during one of the recent periods of 
geological history, arctic conditions extended down into tem- 
perate latitudes. The north temperate zone has just emerged 
from this period ; and during its existence, sheets of ice, form- 
ing great continental glaciers, extended down into regions 
now densely populated. Northern Europe and northeastern 
United States were covered by these glaciers (see Chap- 
ter XVII.). At about the same time that this ice sheet 
extended over northeastern United States, the climate of 
parts of the Great Basin region of the West was transformed 
from an arid condition to one of relative humidity ; and, 
during this time, great lakes existed where now there are 
only desert plains and salt lakes. 

Perhaps the causes for these changes in climate are to be 
found in conditions which we do not as yet understand ; but 
they may in part be due to variations in the movements of the 
earth about the sun. These are too difficult for simple state- 
ment ; but they depend upon slow changes in the distance 
between the sun and earth, and upon the rotation of the 
earth's axis, known as the precession of the equinoxes. 1 

1 The teacher will find this theory fully stated in Croll's "Climate and 
Time," to which reference is made at the end of this chapter. A shorter, 
but very clear statement of the theory, will be found in Geikie's " Text-book 
of Geology," pp. 23-30. 



134 PHYSICAL GEOGRAPHY. 

Since climate varies so remarkably with differences in the 
elevation of land, or in the relation between land and water, 
it is possible that changes of a purely geographic nature may 
account for some of the variations. If large areas of land 
should be raised to greater elevations, or considerable tracts 
be lowered, or if the ocean currents should have their courses 
decidedly changed, the climate of parts of the earth would 
be very different from the present. Such changes actually 
have occurred, and in this way some of the climatic variations 
may be explained ; but at present, only hypothesis can be 
offered to account for the change. 



REFERENCE BOOKS. 

Greely. — American Weather. Dodd, Mead & Co., New York, 

8vo. $2.50. (Valuable information, particularly relating to United States.) 
Abercromby. — Weather (International Scientific Series). Appleton & Co., 

New York, 1887. 12mo. $1.75. (Refers more particularly to Europe.) 
Blanford. — Climate and Weather of India. Macmillan &Co., New York, 

1889. 8vo. $3.50. 
Woeikof. 1 — DieKlimate der Erde. Gostenoble, Jena, 1887. 8vo. 22 m. 
Hann. — Handbuch der Klimatologie. Englehorn, Stuttgart, 1883. 8vo. 

15 m. 
Croll. — Climate and Time. Stanford, London (Appleton & Co., New York 

agents). Fourth edition, 1890. 12mo. $2.50. 

A series of publications on the climate of the states and territories included 
within the arid belt of the West contains much valuable information. It is by 
Greely and Glassford, and was printed by the Signal Service at Washington. 

There is an admirable discussion of some of the climatic features of 
New York, by Turner, in the "Fifth Annual Report of the New York 
Weather Bureau," 1894. This is distributed free of cost, and application for 
it should be made to the director, Professor E. A. Euertes, Ithaca, New York. 

1 In most cases reference is not made to works published in languages other than the English ; 
but these books are of especial importance. For a much fuller bibliography of the literature, 
reference may be made to the author's larger book, now in preparation. The present lists are 
intended to do no more than to refer to a few standard books in which reliable information may 
be found upon the several subjects which of necessity are very briefly treated in this book. 



CHAPTER VIII. 

GEOGRAPHIC DISTRIBUTION OF ANIMALS AND PLANTS. 

General Statement. — There are three great zones occupied 
by life, — the air, the water, and the land. None of the 
animals of the air exist in that medium alone, but they are 
in part terrestrial or aqueous, — largely the former. Aerial 
animals belong to several groups of the animal kingdom, but 
for the most part are either birds or insects. On the land, 
nearly all the great groups a^e represented, though some of 
the truly aqueous animals are absent. In the ocean, the 
fishes and lower forms of animals are predominant, though 
there are groups of birds that dwell on the ocean for 
a greater part of the time ; and some groups of mammals, 
such as the whales and seals, have adopted this zone for 
their home, although nearly all of their fellow-mammals 
live on the land. The group of reptiles is also represented 
in the sea, but the land is their main home. There is 
much difference between the life in fresh and salt water 
bodies, the main life characteristics being the same, but with 
many noticeable minor differences. For instance, insects are 
common inhabitants of the lakes and rivers, but are nearly 
absent from the sea ; and the plant life of lakes is much 
more varied and high in type than that of the ocean, m 
which only a few species allied to the land plants are known 
to occur. 

The Ocean. 1 — The zones of air and land maybe classed 

1 See also Chapter IX., pp. 168-174. 
135 



136 PHYSICAL GEOGBAPHY. 

together ; but the ocean is so different that it must be con- 
sidered separately. The mobility of the water, and the 
moderate temperature ranges over the greater part of the 
ocean, are both favorable to the widespread distribution of 
marine life ; and thus in great oceans we find some species 
ranging almost from one end to the other. The limit of 
temperature is the main check to the spread of ocean animals, 
and this is well illustrated by the distribution of reef -build- 
ing corals, which are practically excluded from zones where 
the water temperature descends below 68°. Because the 
temperature of the ocean water descends as the depth in- 
creases, the forms of life change with the depth. 

In the case of ocean plants, as indeed those of fresh-water 
bodies, depth is a very important factor in limiting distri- 
bution. Below a depth of 200 feet in the sea there is 
a practical absence of any form of vegetable life, because 
below this limit the sunlight is not powerful enough to 
perform the work which plants demand of it. The plants 
of the ocean are partly floating seaweed, so well illustrated 
in the gulf weed or sargassum, which drifts in the Gulf 
Stream, and accumulates in great areas, or Sargasso Seas, 
in the eddies within the whirl of the oceanic currents. 
Many of the plants are attached along the shore line, and 
the most favorable place for this is the rocky shore, which 
furnishes a firm base for attachment. Therefore, along rock- 
bound coasts, the area between tides is covered with a mat 
of seaweed (Figs. 89 and 204). Another favorable place for 
oceanic vegetation, is in quiet, partly enclosed arms of the 
sea, away from the reach of the waves. Here many forms 
of plants exist, some of them belonging to the true grasses ; 
and in such places these help to build level, swampy plains, 
known as salt marshes (Fig. 206). 

Among the most striking features connected with the 



GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 137 

oceanic life, are the wide distribution of its species and the 
great abundance of individuals. A striking difference is 
noticed between the animals existing in the warm waters of 
the tropical belt, and those occurring on the storm-beaten 
coasts of the cold temperate and arctic zones. The latter 
appear hardy, while the former are often exquisite in their 
beauty of color and delicacy of structure. There is much 
less difference in this respect among the inhabitants of the 
mid-ocean ; for here the changes in physical conditions are 
less marked. 

Fresh Water. — In fresh-water bodies there is much less 
variety of life, and usually there is not much opportunity 
for the study of distribution. Land animals, notably insects, 
often go to these water bodies for purposes of breeding ; and 
in many cases marine fishes enter fresh water for the same 
purpose. In certain cases, owing to some accident, these 
ocean animals find their place of outlet cut off, and they 
become land-locked. As a result of this, we sometimes find 
true ocean fishes living in lakes. Sometimes fresh-water 
lakes become transformed to salt lakes, and this change 
gradually exterminates the animals. Finally, these water 
bodies may become dead seas in which practically no life 
exists, as is the case in the Great Salt Lake and the Dead Sea 

The way in which lakes become inhabited is mainly by 
the migration of life along the streams, or else by the 
entrance of land animals ; and if a pond is made in the 
course of a stream, it will soon become stocked with both 
animal and plant life. 

The Land. Effect of Temperature and Moisture. — On the 
land, one of the main factors determining distribution of life 
is that of temperature. As a result of this, we find very 
great differences between the faunas and floras of the tropics, 
and those of arctic latitudes. This difference affects both 



138 



PHYSICAL GEOGRAPHY. 



variety and abundance ; for while there are many vicissitudes 
in the colder zones, everything favors the development of life 
near the tropics. The animals of the Arctic must prepare them- 
selves for the long, cold winter, and at all times the condi- 
tions surrounding them are severe. Only a few forms of 
mammals exist there, and these are very hardy and well pro- 




FlG. 60. 

Near the timber line, Colorado. 



tected with fur. Many of the mammals, and most of the birds, 
leave the coldest part of the zone during the winter; and some 
of the birds that spend the summer within the Arctic circle, 
in winter pass southward to the southern portion of the 
temperate zone. Reptiles are nearly absent from this cold 
region, and the few that do exist there are of small size. In 
summer, the land is clothed with vegetation up to the limits 



GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 139 

of perpetual snow ; but there is a very marked difference 
between the scanty flora of the Arctic and the luxuriant, 
almost impassable tropical forest. Within the Arctic, the 
trees are prevailingly evergreens, and near the snow fields 
all trees disappear, while their place is taken by shrubs and 
the smaller forms of plant life. 

Intermediate conditions exist within the temperate belt. 




Fig. 67. 
Above the snow line, Mt. St. Elias, Alaska. 



There is a great variety of plant and animal life, in the 
southern part merging into the tropical conditions, in the 
northern portion assuming arctic characteristics. Many of 
the birds of the colder parts of the zone migrate to the south 
in the winter ; but the insects, reptiles, and many of the 
mammals, spend a part of the winter in a torpid or dormant 
condition, in this respect resembling the winter behavior 
of plants. They have adapted themselves to the annual 



140 



PHYSICAL GEOGBAPHY. 



climatic change from the rigorous winter to the warm 
summer. 

The influence of temperature upon the abundance and 
kind of life, is also illustrated in the ascent of mountains 
(Fig. 66). Within the tropics, one may pass upward into 
places where plants exist, which in many respects resemble 
those of the temperate zone ; and in this zone a flora of 
arctic habit exists upon many of the highest mountain tops. 
In studying the distribution of animals and plants, altitude 
is found to be a very important factor ; and as one ascends 
o a mountain range, 

he finds familiar 
plants and animals 
disappearing one by 
one, and their place 
only partly taken by 
species which rapid- 
ly decrease in num- 
ber as the ascent 
continues. When 
the snow line is ap- 
proached (Fig. 67), 
the limit of plant life is practically found, although in favored 
places some few species may extend even above this line. 
Before the snow line is reached, one passes the timber line, 
and goes from the forest-covered slope to one on which trees 
do not grow (Fig. 66). The elevation at which this is 
reached, varies with the latitude (see Fig. 65~), and even with 
the portion of the mountain. If one side is exposed to cold 
winds, the limit is reached at a lower altitude than on the 
opposite side ; and the same is true of the side of the moun- 
tain which receives the least sunlight, for in such places the 
average temperature is lower than on the sunny side (Fig. 68). 




Effect of sunlight in raising the zone of vegetation 
higher on the southwest than the northeast side 
of a mountain. 



GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 141 



The moistness of the climate also affects the spread of 
animals and plants ; and in absolute deserts we find an 
almost entire absence of life, while in those arid regions 
which are not true deserts, the plant and animal life are 
peculiar, for the conditions are very unfavorable (Figs. 69 
and 70). Reptiles and a few insects, mammals, and birds 
constitute the fauna ; and the flora is characterized by 
stunted, spiny bushes, and a brown grass that becomes 
transformed to hay as it grows in the dry atmosphere. 
Here the cactus thrives, 
and other prickly and 
unusual forms of vegeta- 
tion exist. With abun- 
dant moisture, vegetable 
and animal life flourish, 
and this is one of the 
reasons for the luxuriance 
of the tropical forests; 
and with abundant plant 
growth, animal life also 
becomes abundant. How 
marked this effect is, can readily be understood by consider- 
ing the difference between the sandy wastes of the Sahara 
(or the arid regions shown in Figs. 69 and 70) and the 
tropical forests in the same latitudes (Fig. 71). While 
these are extremes, even slight differences in rainfall will 
cause marked changes in life. 

Plant and Animal Habits. — Aside from those differences 
among animals and plants upon which the zoological and bo- 
tanical classifications are based, there are certain differences 
in habit which are of some interest from the present stand- 
point. Plants are for the most part fixed in a definite 
place, and the opportunity of distribution comes only from 




Fig. 69. 
Arid land vegetation. 



142 



PHYSICAL GEOGRAPHY. 



the seeds. Therefore in the study of geographic distribu- 
tion of plants, the seeds are of much interest. Some are 
heavy, and these drop to the ground close by the tree ; but 
in some cases, these heavy seeds are enveloped in a fruit, 
which is eaten together with the seed ; and since the germ is 
often protected by an indigestible shell, the vital part of 
these seeds may be carried for long distances, and then be 
left upon the ground unharmed. Some seeds cling to the 




Fig. 70. 
Arid land vegetation in the Canon of the Bio Grande, northern New Mexico. 



fur of animals and are thus distributed, and many are 
drifted to distant regions by the winds. In these, and other 
ways, plants are spread from one place to another. 

Among land animals, there are great differences in habit. 
Some move slowly, others rapidly, and some are able to fly 
in the air. Most animals of the land dwell on the surface ; 
but for a part of the time, many make their home either 
in the air or in the water, and some spend a part or all of 



GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 143 

their time beneath, the surface of the ground. Naturally, 
because of these variations, there is much difference in the 
distribution of animals, and in the means by which they are 
distributed. 

Life Zones. — As a complex result of these animal and 
plant peculiarities, together with their physical surround- 
ings, and certain inherent conditions which cause life to 
grow and develop, there has resulted a peculiar distribu- 




Fig. 71. 
The tropical forest. 



tion of life on the land. The most perfect adjustment to 
conditions is noticed upon the connected continents ; and 
here we find three great zones, the tropical, temperate, and 
arctic, in each of which there are sub-zones which are due to 
irregularities in climate or in topography (Fig. 72). There 
are mountain and desert irregularities, as well as others. 

Not only do these zones exist upon the several continents, 
but quite different species, both of animals and plants, char- 
acterize the separate continental areas. The plants and ani- 



144 



PHYSICAL GEOGRAPHY. 



mals of Europe are quite different from those of America ; 
of South America from Africa. 1 Yet there is remarkable 
uniformity in the fact that the same large groups are pres- 
ent in each, as if by some means there had been an occa- 
sional connection or communication. Evolution teaches us 
that animals and plants have been developing from simpler 




Fig. 72. 

Diagrammatic representation of the life zones of the United States, showing 

influence of latitude and topography. 

to higher forms, and we now know considerable concerning 
the steps along which this has proceeded. The fact of the 
difference between the life of the several continents, shows 
that there has not been constant connection ; but the resem- 
blances prove that there has been some communication. 

1 To illustrate, we have temperate and tropical zones in both South America 
and Africa, and in each of these also the subdivisions of coast, desert, 
and mountain belts. But the tropical forest of Africa bears only a general 
resemblance to that of South America. However, these resemble each other 
much more closely than do the forests of tropical and arctic zones. 



GEOGBAPHIC DISTRIBUTION OF ANIMALS, ETC. 145 

Even more strikingly is this proven by the resemblances 
and differences between the life of the oceanic islands and 
that of the continents. These land bodies are separated, 
and in some cases have always been separated, by a great 
ocean barrier ; yet at times, as for instance in the Bermudas 
and the West Indies, the life of the islands resembles that 
of the neighboring mainland, although numerous species are 
absent. On the other hand, there are many cases in which 
the insular life is entirely unlike that of the mainland. For 
instance, the only native mammals of New Zealand, are two 
species of bat, which have been able to spread themselves by 
means of flight. Other vertebrate animals are scarce on this 
land, and in most cases the animals are of peculiar types. 

The animals of the East Indies are quite like those of 
Asia; but Australia, which lies only a short distance south 
of these, forms a perfectly unique life zone. Excepting a 
few species of bats, rats, and mice, all of the mammals 
belong to peculiar types, such as the kangaroo group, and 
the group to which the duck-bill belongs, — an animal which 
combines the habits and characteristics of mammals with 
that of laying eggs. The birds are also peculiar, including 
among their number many king-fishers, parrots, birds of 
paradise, and other peculiar forms. 

The Spread of Life. — The prime cause for the wide dis> 
tribution of land animals is found in themselves, as a result 
of voluntary movement from one place to another. Many 
birds may easily pass for long distances, and in this way 
they may reach far-distant lands ; but usually they are con- 
tent to stay in their normal home. 

During storms they may be blown far from their home, 
and when they alight, it may be upon some distant island, or 
in some other place from which return is not easy. Ships a 
hundred miles from shore, often serve as a resting place for 



146 PHYSICAL GEOGRAPHY. 

some land bird which is lost on the open sea. In the water 
there are floating logs which may serve as resting places, 
and in this way flying animals may be distributed. A few 
years ago, during a violent storm, a sea gull fell exhausted 
not far from Ithaca, New York, at a distance of two or three 
hundred miles from its ocean home, which was certainly not 
north of Philadelphia. Naturally, because of this aid of the 
wind, winged animals are most widely distributed. 

Land animals that cannot fly, distribute themselves by 
moving over the land, each generation pushing its frontier 
line farther than the preceding, provided other conditions 
are favorable. As is stated in the next section, there are 
certain limitations to this natural spread of life. 

In a second way, land animals may be distributed by 
accidental means. Drifting in the ocean currents, there 
are often logs of wood which have floated down some river 
to the sea. Tree trunks from the American coast are in this 
way borne to the European coast and there stranded. Ani- 
mals may be carried upon these, and if they survive the 
journey, they may begin to increase on some land remote 
from their old home. This is particularly likely to happen 
to animals like reptiles, which can live for a long time with- 
out water or food, or to insects which are in the egg or in 
the cocoon. In rare cases, even the higher types of life may 
pass through such a journey ; but such animals must be 
small, for only these can be thus floated. It is for these 
reasons that the large mammals are so rarely found upon 
the islands of the ocean, even though the distance to the 
mainland is slight. 

By a change in climate, such as that which produced the 
glacial period, animals may be forced to migrate, and in so 
doing they may cause other animals to abandon their homes. 
When the ice enveloped the northeastern United States, the 



GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 147 

nimals were either killed or driven away into the southern 
,nd more favorable regions. The effect of this migration 
Qust have been felt far from the ice front ; and there are 
till signs of its influence in the distribution of animals 
,nd plants in eastern United States. 

Barriers to the Spread of Life. — The great barrier is the 
»cean. More effectually than any other feature of the earth 
t serves as a check to the spread of animals and plants. In 
he case of Australia, it has served as an effectual check 
ipon the spread of the large and powerful animals of the 
£ast Indies, and in a less perfect way, even upon the more 
fasily distributed forms of life. Short arms of the sea are 
iot so effectual, but even these serve as a partial barrier, 
rhe study of the problem offered by the Australian fauna, 
eads to the conclusion that this continent has not been con- 
lected with the Asiatic lands since the higher animals began 
:o exist ; and in other parts of the world, a study of the dis- 
;ribution of life, proves that some of the ocean barriers of the 
)resent have not always existed. 

Next to this great oceanic barrier, the most important 
)bstacle to the spread of life is probably to be found in high 
nountain chains, such as the Andes and the Rockies. Many 
mimals and plants are completely checked by these. Nearly 
;he same is true of deserts ; for if it is not possible to pass 
iround these, many species find it impossible to pass from 
me side of them to the other. In some cases even large 
?ivers serve as a boundary line, separating a zone occupied 
3y a species from one in which it is absent. 

Effect of Man. — The above remarks hold only for the 
natural distribution. Now man has come upon the scene as 
i disturber of the natural order, and everywhere in the 
world we see the result of his interference. We have 
European and Asiatic trees in the garden, and, in some places, 



148 PHYSICAL GEOGRAPHY. 

even in the forests. There are foreign weeds in the field, 
foreign birds, insects, and mammals (notably the rat), as 
pests, or as unnoticed additions to the flora or fauna. The 
ancient marsupials are no longer the most important mam- 
malian possessors of the Australian zone, but man has caused 
an invasion of their territory. 

Man is killing here and adding there, with the result that 
intentionally or unintentionally, he is changing the life zones ; 
but while thus interfering with the natural spread of life, 
and, in some cases, succeeding in domesticating plants and 
animals of one zone to the conditions of another, he is not 
able to disturb the great divisions of tropical, temperate, I 
and arctic, of mountain and desert. These depend upon i 
physical conditions of too fundamental importance. The 
camel may be domesticated in the desert of southern Cali- 
fornia, but it cannot thrive in New England ; the tiger 
might be introduced into South America, but not into Scan- 
dinavia ; the palm of the central Pacific might be made to 
grow on the islands of the central Atlantic, but not on the 
slopes of the Rocky Mountains. Thus while man will 
greatly aid in the distribution of animals and plants, in 
general he will succeed only in disseminating them over i 
zones in which the prevailing conditions are similar. 



REFERENCE BOOKS. 

Wallace. — Island Life. Macmillan & Co., New York, second revised 

edition, 1892. 8vo. $1.75. 
Wallace. — The Geographical Distribution of Animals. (Vols. I. and 

II.) Harper & Brothers, New York, 1876. 8vo. $10.00. 



Part II. 

THE OCEAN. 



CHAPTER IX. 

FORM AND GENERAL CHARACTERISTICS OF THE OCEAN. 

Distribution of Land and Water. — A glance at a globe 
shows a very marked irregularity in the distribution of land 
and water in the different hemispheres. It is possible to 
divide the earth into two hemispheres, in one of which there 
is little land, while in the other the water area is small 
(Fig. 2). Nearly three-fourths of the earth's surface is 
covered by water, the total area of water surface being about 
145,000,000 square miles. Land rises from the water in 
the form of continents and islands, which differ greatly in 
outline and topography. 

Composition of Ocean Water. — The ocean is between 96 
and 97 per cent pure water, the remainder being divided 
between several salts, of which the most abundant is com- 
mon salt. In addition to this common salt, there is an 
appreciable amount of chloride of magnesium, carbonate of 
lime, some sulphates, and very minute quantities of other 
substances. Probably some compound of every known 
element is dissolved in the ocean, in such minute quantities 
that they can be detected only by the most careful analysis. 
In addition to these slight impurities, the water has absorbed 
a considerable amount of atmospheric gases. It is upon this 
that the ocean life depends. 

In different parts of the world, there is a considerable 
variation in the percentage of salt impurities, the range 
being between 3.3 and 3.73 per cent. At the same time 

151 



152 PHYSICAL GEOGRAPHY. 

with this change in amount of salt, there is a variation in 
the density of the water. Representing fresh water as 1, 
the average density of sea water is 1.026. There are many 
reasons for variation in the salinity of sea water. Where 
rivers enter the ocean, the density is decreased by the addi- 
tion of fresh water ; and also where rains are abundant, as 
they are in the belt of doldrums, the surface water has its 
density decreased. On the other hand, where evaporation is 
great, the removal of the fresh water tends to concentrate 
salts and therefore to increase the density. In the Mediter- 
ranean and the Red Sea, the ocean water is relatively dense ; 
and the same is true of the belts of ocean water over which 
the dry trade winds constantly blow. 

Color and Phosphorescence. — The color of the ocean i? 
naturally blue. This is partly due to the fact that the blue- 
ness of the sky is reflected upon the water surface, and partly 
to the scattering of light rays which enter the water, this 
cause being analogous to that of the blue color of the sky 
itself. The color of the bottom often imparts to the water 
a different shade from the typical blue of the ocean ; and 
where the water is shallow, green shades are often produced. 
The Red Sea owes its color to the presence of many minute 
forms of vegetation, belonging to the group of Algse, while 
the color of the water near some coasts is due to the pres- 
ence of great quantities of mud brought down by the river. 

At times, particularly on quiet nights, the ocean waters 
are aglow with a silvery gleam of light, which is known as 
phosphorescence. It is similar in origin to the glow of the 
fire-fly which we see on warm summer nights. In the sur- 
face waters of the ocean, there are countless millions of 
microscopic animals, nearly all of which are able to emit 
a tiny spark of this strange light ; and their power to do 
this seems to vary from time to time. Therefore on some 



GENERAL CHARACTERISTICS OF THE OCEAN. 153 

nights the surface is free from this light, while at other 
times every ripple causes a silvery gleam. In rowing upon 
the surface of the sea at such times, a trail of light follows 
behind the boat, and drops of gleaming water fall from the 
tips of the oars. 

Exploration of the Ocean Bottom. — It is only recently 
that the bottom of the ocean has attracted much attention. 
Until thirty years ago, it was supposed that after passing 
below a depth of a few hundred feet, the bottom of the 
ocean was a great, uninteresting desert. And thus, until 
that time, we were almost entirely ignorant of the condi- 
tions existing on more than one-half of the earth's surface. 
To the naturalists of the time it seemed absolutely impossi- 
ble that life could exist in the depths of the sea. 

When oceanic cables were laid, the beginning of the study 
of the deep sea was made, and proof was soon obtained that 
animals did live in the great ocean depths. This proof first 
came from the Mediterranean, where a submarine cable was 
drawn to the surface for repair. Attached to it were a 
number of animals, which therefore must have lived where 
the cable lay ; and the depth of water at this place was 
much greater than the supposed limit of life. The fact 
that conditions favoring the development of animals proba- 
bly existed over the entire ocean bottom, immediately 
created a desire for exploration ; and to this scientific inter- 
est was added the practical one, which resulted from the 
necessity of obtaining a knowledge of the physical features 
of the bottom, in order to make more easy the extension of 
oceanic cables ; and soon governments began the study of 
the ocean bottom. 

Methods Used in Deep-sea Explorations : Sounding. — In a 
study of the ocean bottom, we wish to discover something 
concerning the life that exists there, something about the 



154 



PHYSICAL GEOGRAPHY. 




topography, and something concerning the kind of bottom, 
as well as the character of the water, and the various physical 
conditions. For this purpose, one thing is of prime impor- 
tance, namely the depth ; and in every deep-sea exploration 
this is the first fact obtained. 1 For this sounding, many 
ingenious contrivances have been invented, the one best 

adapted to deep-sea work be- 
ing the Sigsbee deep-sea sound- 
ing machine (Fig. 73). A 
weight attached to the end of a 
fine steel wire, is carefully low- 
ered until the bottom is reached. 
The ball of the sounding ma- 
chine sinks by its own weight ; 
and when it touches bottom 
a jar is sent through the wire, 
which is felt even at the sur- 
face. The entire machine is 
very delicately constructed, and 
it records ocean depths with 
great accuracy. The wire 
used is so fine that it would be 
impossible to draw the weight 
back to the surface, and the 
instrument is so contrived 
that this is left behind when 
it touches the bottom of the ocean (see Fig. 73). 

The weight is nothing but a cannon ball through which a 
hole had been bored. Into this hole is placed a cylinder, 
which remains open during the passage of the weight to the 
bottom, and which is automatically closed when the line is 
drawn in. Usually the bottom of the cylinder is covered 
1 Ocean depths are measured in fathoms, a fathom being six feet. 




73T 
Fig. 73. 
Deep-sea sounding machine, with and 
without the sinker. 



GENERAL CHARACTERISTICS OF THE OCEAN. 155 



with wax or soap, to which, clings a sample of the mud of 
the ocean floor ; so that as the instrument is drawn to the 
surface, we have both water and mud from the bottom. 

Near the weight a thermometer is attached to the line ; and 
this is so made that it is inverted 
when the wire is reeled in, and an 
automatic register of the tempera- 
ture at the time of inversion is thus 
made. Very often several ther- 
mometers are attached to the line 
at different distances, so that we 
obtain a knowledge of the tempera- 
ture of the ocean water from the 
surface down to the very base of 
the column. 

Dredging. — In order to obtain a 
knowledge of the kind of life that 
exists in these great ocean depths, 
another method, that of dredging, 
must be followed. The dredge, or 
deep-sea trawl (Fig. 74), is an iron 
frame several feet in length, to 
which is attached a bag net. This 
is lowered to the bottom and 
dragged over it, usually for several 
hours. The sounding apparatus is 
lowered perpendicularly ; but the 
dredge is lowered to the bottom, 
and then more rope is reeled out, 
so that it may be kept upon the 
bottom and dragged over it. This is done partly by attach- 
ing weights to the dredge, and partly by the natural sagging 
of the wire rope. After the dredge has been down for a 




Fig. 74. 
Deep-sea trawl. 



156 



PHYSICAL GEOGRAPHY. 



sufficient length of time, it is drawn to the surface and its 
contents examined. 

Imagine a balloon sailing through the air at a height of 
three miles or more, and dragging a frame a few feet in 
length, over a distance of a few miles. If the operators of 
this apparatus should imagine that, as a result of a few trials, 
they had obtained a fair knowledge of the life existing on 
the surface of the earth, it will readily be seen that they 
would be very much mistaken. All swiftly moving animals 
would escape, and only those would be taken which were 
small enough to enter the dredge, and so slow that they 
could not escape from it. In a measure this is true of our 
explorations of the deep sea. If large animals exist there, 
our methods of exploration are not calculated to discover 

them, nor should we 
expect to obtain 
many animals that 
are capable of rapid 
movement. 

Topography of the 
Ocean Bottom : Gen- 
eral. — There is a 
very profound dif- 
ference between the 
outline of the ocean 
bottom and the fea- 
tures of land, as we 
know them on the 
continents. In both 
places the crust of the earth is subjected to a tendency 
to wrinkle, and therefore to form mountain folds ; and 
in both cases also, volcanoes are produced. But on the 
land, there are forces at work which are absent from 




Fig. 75. 

Diagram contrasting land and ocean bottom topog- 
raphy, a, a, a, land surface ; B, B, height to 
which mountain would rise if denudation were 
not acting; c, c, undulating ocean bottom; d, d, 
ocean sediment partly obscuring mountain fold 
E, E ; V, volcanic cone. 



GENERAL CHARACTERISTICS OF THE OCEAN. 157 

the ocean. The rain, rivers, changes in temperature, and 
wind, are engaged in the combined action of carving and 
sculpturing the land, the result of which is to make the 
surface very irregular, and at the same time to gradually 
lower it (a, a, Fig. 75). None of these tendencies exist in 
the ocean. 

The oceanic areas are the gathering grounds for the waste 
of the land. Materials worn from the continents are borne 
to the sea in rivers, or are wrested from the land margin by 
waves, and distributed over the sea bottom. Materials car- 
ried in solution by river waters also find their way to the 
ocean ; and from these the animals that dwell in the sea, are 
able to take the materials which they build into their skel- 
etons, and which upon death they leave as a contribution to 
the ocean floor. Therefore the tendency of deposition in 
the ocean is to smooth the surface. Thus in the sea, while 
excessive elevations are occasionally found, the general topog- 
raphy is remarkably uniform. There are great elevations, 
because nothing is present which tends to destroy the diver- 
sities produced ; but the absence of the agents that are carv- 
ing and sculpturing the land, makes the sea bottom a place 
of great regularity. 

In the ocean, there are prevailing conditions of great, wide- 
stretching oceanic plains or plateaus ; and where there are 
elevations, these are usually so gentle that they would appear 
to be nearly level. Occasionally, where volcanic peaks rise 
in the ocean, we find exceptionally steep slopes. The agents 
of the air are not present to carry away the materials which 
are building the cone, and therefore most of the material 
that is ejected is piled into one mass. 

In a distance of about 70 miles from Porto Rico, the depth 
of the ocean descends to 4561 fathoms ; and in this region 
there is a difference in elevation of fully 30,000 feet in a 



158 



PHYSICAL GEOGRAPHY. 



1 



distance of about 80 miles. Within sight of the Bermudas, 
at a distance of from 10 to 30 miles from 

_ land, the ocean reaches a depth of from 

■J 2700 to 2900 fathoms. Among the oceanic 

t> islands of the Pacific, differences in eleva- 

s tion fully as great as these are frequently 

| discovered. On the land there are no 

g, such excessive differences in elevation as 

® those which exist among the volcanic 

.§ islands of the ocean. 

•g The Atlantic Ocean. — Perhaps the best 
m way to obtain an idea of the topography 
J of the Atlantic Ocean, is to make a see- 
's tion across it, following approximately 
« "S the line traversed by the oceanic steamers 
U f, (Fig. 76). Starting from the shore of 
^ ^ to New York, an even, gently sloping plain 
2 r « is found stretching eastward to a distance 
-g +| of from 50 to 75 miles. It is almost 
®" go level, and its features are quite like some 
| of the very level plains on the land. This 
A plain extends far above the mouth of the 
^ St. Lawrence, including nearly all of the 
3 area between the present New England 
° coast and a line about 100 miles from 
a the shore. South of New York this sub- 
^ marine plain, or continental shelf, rapidly 
© narrows until off the Carolina coast it is 
.2 a very narrow strip. Such a continental 
S shelf as this, is found along the bordo^ of 
o nearly every continent on the earth, 
though in width there is much variability 
(Plate 14). 



GENERAL CHARACTERISTICS OF THE OCEAN. 159 

Passing eastward, and for a while leaving the track of the 
ocean steamers to the northward, a region of very rapid 
slope is encountered. This is known as the continental 
slope, and in many places the rate of its descent is as great 
as that of a mountain. In a distance of a few miles, one 
passes from the edge of the shelf, whose depth is usually 
about 100 fathoms, to oceanic depths as great as 1000 
fathoms. After the 10 00 -fathom line is reached, the exces- 
sive rapidity of the slope decreases ; but still the ocean depth 
rapidly increases to 1500 or 2000 fathoms. In a distance of 
from 50 to 100 miles, the depth has increased from 100 to 
2000 fathoms, where the true oceanic plateau is reached. 

Almost the entire ocean is included in this deep plateau 
area. Extending northward and southward, to' the Arctic 
and the Antarctic circles, there is a monotonous, level plain, 
with ocean depths varying between 1000 and 3000 fathoms, 
and only rarely broken by some slight interruption. 

Passing eastward, this plateau extends just beyond the 
middle portion of the ocean, where the bottom gradually 
begins to rise, forming the Mid-Atlantic Midge. It extends 
with considerable uniformity from Iceland to the southern 
limit of the Atlantic Ocean ; but it reaches the surface only 
here and there, as in Iceland, the Azores, St. Paul, Ascen- 
sion, and Tristan da Cunha. It is not a continuous ridge, 
but an elevated portion of the ocean bottom, whose broad 
crest now reaches the surface, and again is fully 1000 
fathoms below it. Almost everywhere along this area, the 
ocean depth is less than in other places far from land. 

After passing the crest of this rise the depth again 
increases, until soundings of over 2000 fathoms indicate 
another approach to the great submarine plateau. The 
plateau on the eastern side is less extensive than that on the 
western ; and as the European coast is approached, the deep 



160 PHYSICAL GEOGRAPHY. 

oceanic plateau rises toward the continent. Here the con- 
ditions that were noticed off the American shore are prac- 
tically repeated. There is a slope and then a continental 
shelf, which merges into the continent itself. In the vicinity 
of the British Isles the shelf is broader than it is along the 
coasts of France and Spain. 

Other Oceans. — Much less is known concerning the con- 
ditions in the depths of the Pacific ; and almost nothing is 
known concerning the Arctic and Antarctic oceans. So far 
as our knowledge of the Pacific and Indian oceans warrants 
any definite conclusions, we may say that the conditions of 
the Atlantic are in a general way repeated. The great 
monotonous plain is more broken by volcanic peaks ; and a 
greater depth is found in the Pacific than in any other part 
of the ocean (see Plate 14). Depths greater than 4000 fath- 
oms have been discovered in several places ; and in one place, 
near the Kurile Islands, a sounding of 4655 fathoms was 
made. The deepest place in the Atlantic (4561 fathoms) is 
near Porto Rico. It is a noticeable fact that these excessive 
depths are found close to the land. While the greatest ele- 
vations occur on the land, the average oceanic depth is very 
much .in excess of the average land elevation 1 ; and the great 
land elevations are at a considerable distance from the sea, 
so that the elevation of the high mountain peak above its 
base is much less than its elevation above sea level. The 
greatest ocean depths descend almost directly from the land. 

Topography near the Coast. — While this description of 
the ocean bottom will serve to present the features of the 
deep sea, it does not convey any idea concerning the irregu- 
larities near the coasts. Along all continent margins, and 
particularly among archipelagoes, the form of the bottom is 

1 The average depth of the ocean is as much as 12,000 feet, while the 
average elevation of the land above sea level is not much more than 2000 feet. 




H 


o 






hi 


crt 


Ph 






■d 




+3 




Ml 




fl 



162 PHYSICAL GEOGBAPHT. 

exceedingly irregular. Without entering into the subject 
in very great detail, these irregularities could not be ade- 
quately described ; and indeed, our knowledge of the larger 
part of the ocean floor is so slight, that as yet we know only 
the general features. 

Temperature of the Ocean Bottom. — In the neighborhood 
of continents, where the depths of the sea are relatively 
slight, the temperature is more or less irregular, and deter- 
mined by local conditions. It changes with the season, and 
is influenced by the oceanic and tidal currents, and even by 
the prevailing winds. 

After passing this shallow and variable zone, very uniform 
temperature conditions are encountered. As a general state- 
ment, it may be said that throughout the ocean, there is a 
decrease in temperature with the increasing depth. Starting 
from the variable zone of relatively high temperatures, there 
is at first a rather rapid descent until the zone of about 40° 
is reached at a depth of a few hundred fathoms. After 
this, there is a very gradual descent in temperature (Figs. 76 
and 83), until the cold becomes as great as that of fresh 
water at the point of freezing. Over a very large part of 
the ocean bottom the temperatures are between 32° and 35°. 
On some parts of the ocean bottom, particularly in the South 
Atlantic and the South Pacific, temperatures of 32° and 
even of 31° are found. 1 

While this is true as a general statement, there are numer- 
ous exceptions to it. In the Mediterranean (Fig. 77), there 
is a decrease until the level of the bottom of the Straits of 
Gibraltar is reached, after which the temperature remains 
uniform, while in the Atlantic there is a normal decrease. 

1 In this connection it must be remembered that the freezing-point of salt 
water is lower than that of fresh water ; and therefore temperatures lower 
than that which we call the freezing-point may be found in the ocean. 



GENERAL CHARACTERISTICS OF THE OCEAN. 163 

This is because the Mediterranean is enclosed, and its water 
enters over the Straits, and hence with the temperature at 
that level. Also, in the Gulf of Mexico, the temperature in 
the deepest part is only 39^-°, which is the same as that at 
the Straits of Yucatan. The deep part of the Gulf of 
Mexico is 2119 fathoms, while that of the Straits of Yucatan 
is only 1127 fathoms. It will be seen, therefore, that this 
decrease in temperature does not depend upon increase in 
depth. 

The peculiar distribution of temperature in the deep sea, 
is probably due to movements of water on the bottom. In 
the Arctic regions 

the cold water sinks, 1 1 

while at the tropics 
warm water rises : 



Atlantic 


68° 




2 =S ~ 

03 <3 


Mediterranean 

75° 




54° 






55° 




52° 




*S«?*S>y 


55° 




38° 


/■'' 


'■:0;:-; : l 


*\ 55° 




37° 




'.\'.'-/\ 55° 




xyt- 


; -- ;;: : :; k55° 



200 



and there is a con- I 500 

stant passage of 



water from one of 
these zones to the 
other, giving a sur- 



Fig. 77. 



Diagram showing the temperature peculiarities of 
face movement to- the Mediterranean. 

ward the poles, and 

a bottom movement toward the equator. If a barrier exists 
in the line of this deep-sea circulation, the normal decrease 
in temperatures is interfered with. At the Straits of Gib- 
raltar, the water which passes into the Mediterranean does 
not come from the bottom of the ocean, but from a level 
determined by the bottom of the Straits. 

Light on the Ocean Bottom. — It seems certain that sun- 
light cannot possibly penetrate through several miles of salt 
water ; and if this is true, the greatest depths of the 
ocean are practically dark, so far as sunlight is concerned. 
Although it is probable that no sunlight penetrates to these 



164 PHYSICAL GEOGRAPHY. 

zones, it still seems certain that some kind of light does exist 
there. This conclusion is forced: -upon us by the fact that 
many of the animals in the depths of the sea have well- 
developed eyes ; and, further, that many of them are brill- 
iantly colored. Animals living in dark caves become blind ; 
and it seems hardly probable that these inhabitants of the 
deep sea would continue to develop eyes for ages after their 
usefulness had ceased. 

Phosphorescence is a possible source of light on the ocean 
floor. After nightfall, whenever a dredge-load of materials 
is brought from the deep sea to the surface, it is aglow with 
the dull white light of phosphorescence. Each animal, each 
particle of mud, gleams with this light. 

Materials composing the Ocean Floor: Mechanical Sedi- 
ments. — There are two classes of substances spread over the 
ocean bottom : one mainly derived from the land, or j from 
fragments of rock emitted from volcanoes ; the other, from 
animals which have lived in the ocean. The latter covers by 
far the greater part of the ocean floor. The sandy and 
clayey fragments of rock which are derived from the land, 
are spread over the bottom of the sea only in the neighbor- 
hood of the coasts. 

Giobigerina Ooze. — One of the most striking facts con- 
nected with the ocean, is that the floor, covering an area 
greater than one-half that of the entire earth's surface, is 
made up of the remains of minute animals. When seen 
with the unaided eye, the deposit is a blue mud or ooze ; but 
when examined with the microscope, it is found to be com- 
posed of fragments or entire shells of tiny animals, generally 
belonging to the group of Foraminifera. The most abun- 
dant of these are members of the genus Giobigerina ; and 
these are so characteristic of the deposit, that it is known as 
the Giobigerina ooze (Fig. 78). 



GENERAL CHARACTERISTICS OF THE OCEAN. 165 



It covers the greater portion of the Atlantic, and large 
parts of the Pacific and Indian oceans. Its rate of accumu- 
lation must be extremely slow ; 
for although the animals which 
compose it are very abundant 
in the surface waters of the 
ocean, they are so small that 
it must require long periods 
of time to form any considera- 
ble depth of ooze. Each par- 
ticle must depend upon the life 
and death of a tiny animal. 
The chalk of England, and 
other regions, is a rock whose 
origin was similar to that of the 
Globigerina ooze. 

Med Clay. — At a depth great- 
er than 2000 or 2500 fathoms, 
the bottom ooze changes its 
character and becomes known as 
red clay. This form of ocean deposit is particularly abun- 
dant in the Pacific, although it is not entirely absent from 
the Atlantic. It is one of the most remarkable deposits 
being made in the ocean. In these great ocean depths, the 
power of the salt water to dissolve the lime of shells has 
increased until this substance is taken in solution as the 
shells drop from the surface. Therefore the insoluble por- 
tions, of which there are tiny amounts in every shell, are the 
only parts of the Globigerina that reach the bottom. There- 
fore the ooze is in part a residue of the shell after the 
soluble portions have been removed. And if the shells were 
small at the beginning, how much smaller must these tiny 
remnants be! 




Fig. 78. 

Globigerina ooze from the ocean 

bottom. 



166 PHYSICAL GEOGRAPHY. 

It is not exclusively made of the residue of the shells 
of surface animals, but contains contributions from other 
sources. The most common addition comes from pumice 
rocks, which were ejected from volcanoes, and after floating 
for some time settled to the ocean bottom at some distant 
point. Therefore, remnants of volcanic ash or pumice are 
common in the red ooze. Aside from this, there are frag- 
ments of meteorites which have dropped to the bottom, 
indicating exceedingly slow accumulation. This deposit 
covers an area of over 51,000,000 square miles, which is a 
little more than that covered by the Globigerina ooze. Each 
kind of deposit covers an area equal to about one-fourth of 
the earth's surface. 

Life in the Ocean : Pelagic or Surface Faunas. — The 
ocean is the great meeting ground of the life of three 
provinces, — the air, the land, and the water. Forms belong- 
ing to all the great groups of the animal kingdom find it 
possible to live in the conditions which exist in the ocean. 
There the conditions of life are remarkably uniform ; for 
there are few changes in temperature, and few variations such 
as animals on the land experience. Day after day, and year 
after year, the surrounding conditions are nearly the same. 
No such difference exists between the surface faunas of the 
ocean in different latitudes, as between the land animals of 
the tropics and the temperate latitudes. This is partly 
because the temperature of the water changes very slowly 
and very slightly, and it is also in part due to the fact that 
the waters of the ocean surface are in movement, so that the 
temperatures of one latitude are distributed to another. 
From the tropics, the currents bear bodies of warm water, 
and in them animals of tropical origin ; and these may be 
distributed far over the surface of the ocean. 

So uniform are the conditions of temperature, that even 



GENEBAL CHARACTEBISTICS OF THE OCEAN. 167 

very slight differences will cause marked changes in the 
faunas. In the Gulf Stream, which flows at a distance 
of 100 miles or more from the land, there are found many 
creatures of tropical origin, which cannot exist in the 
colder waters near the coast. At times, during strong 
prevailing winds from the south, these creatures are driven 
into the colder waters ; and, as a rule, they are unable to 
survive the change. The ocean surface is particularly favor- 
able to the wide distribution of animals. It is constantly in 
motion, and as a result of this, hardy animals may be distrib- 
uted from one end of an ocean to the other. 

Many of the oceanic animals are free-swimming creatures, 
others are drifting animals, and still others are attached to 
floating objects. This last group is particularly liable to 
be found attached to the floating seaweed or Sargassum, 
which at times, particularly in the eddies between the 
ocean currents, exists in such abundance that these areas 
are known as sargasso seas. All except the largest of the 
surface animals are in a measure at the mercy of the winds 
or currents. 

At the surface, and on the ocean bottom, there is abun- 
dant life. Between the surface and the bottom, over the 
greater part of the ocean, there is a zone of water, at least 
two miles in depth, whose conditions as regards habitation 
are not known. It is the greatest unexplored area on the 
earth, and we are unable to say whether it is a great desert 
region, or whether it is actually inhabited. It is exceed- 
ingly difficult of exploration ; but since animals have been 
found in every explored nook of the ocean, and have become 
adapted to each place, it seems probable that some have 
found this zone and have adapted themselves to it. 

Littoral or Shore Faunas. — Along the shore line, the con- 
ditions more closely resemble those of the land than in any 



168 



PHYSICAL GEOGRAPHY. 



other part of the ocean. There is no such monotony of 
conditions as we find, at the surface of the ocean away from 
the land. But from day to day, from season to season, and 
from place to place, there are very marked differences in the 
conditions upon which the animals depend for their existence 
and variety. Here, as in every part of the ocean, tempera- 
ture is a very important cause for differences in faunas 
and for variation in animal forms. Even a few degrees of 
temperature will cause a very marked difference in the 

abundance and 
variety of ani- 
mal life. This 
is well illus- 
trated on the 
coast of Massa- 
chusetts, where 
the end of Cape 
Cod serves as 
a dividing line 
between two 
quite distinct 
faunas, because 
on the northern 
side of the cape 
the water is 
cool, while on the southern side it is comparatively warm. 
The influence of the Gulf Stream is felt south of Cape Cod, 
while north of it, in Massachusetts Bay, the cold Labrador 
current reduces the temperature. 

Another limitation upon the spread of animals along the 
shore, is that of food supply. Perhaps the best illustration 
of this is found in coral regions. At the points reached by 
food-bringing currents, the abundance and variety of life is 





Fig. 79. 
Coral reef on the Australian coast. 



GENERAL CHARACTERISTICS OF THE OCEAN. 169 

very great (Figs. 79 and 207), and the coral polyps select 
from the water the food that they need. Soon the waters 
are robbed of their food supply, and in passing on are unable 
to support abundant coral growth. It has been noticed 
among the coral reefs, that on one side of a coral bar, 
the polyps grow readily and in great numbers, while on 
the opposite side, they are very scarce and not well devel- 
oped. In the one case there is an abundance of food, in the 
other, the food supply has been taken from the water by 
those corals which have the more favorable situation. 

It follows from this that circulation of water must take 
place in order to bring fresh food supply to the animals 
which are fixed in one place, and which are not able to move 
about for the purpose of obtaining the food which they need 
for existence. Therefore we rarely find coral reefs in other 
places than those bathed by currents. 

The animals that dwell upon the shore line are of several 
kinds : those that are free swimming and able to move 
about ; those that are drifted against the shore by accident ; 
those that crawl about ; those that are attached firmly to 
the rocky coasts ; and those that burrow in the clay and 
sand which are found in certain places. Since animals that 
are in the habit of attaching themselves permanently to one 
place, can find no opportunity for this attachment in places 
where sand and clay form the coast line, it follows that as 
a result of the differences in kind of rock, there may be very 
marked changes in the faunas from one place to another. 
On the rocky coast, the animals are almost entirely of types 
which are attached or which crawl about, while on shores 
that are sandy or clayey, the animals are almost all of the 
burrowing and crawling types. 

Faunas of the Ocean Bottom. — Every dredge load that is 
brought to the surface during deep-sea exploration, proves 



170 PHYSICAL GEOGRAPHY. 

the presence of a great pressure of water in the depths of 
the sea. The more highly organized animals, such as the 
true fishes, are unable to accommodate themselves to this 
change in condition ; and when they are drawn to the 
surface, they are commonly broken by the expansion of 
the gases within the body. Their eyes protrude from the 
head, the air bladder extends from the mouth, and the skin 
is cracked and fissured. Thus while they may live with 
immense pressures upon every particle of the body, they are 
unable to exist when the pressure is removed from the 
outside, while it still partly remains on the inside. 

As a result of deep-sea exploration, it has been found that 
all the ordinary types of marine animals exist on the ocean 
bottom, and that in certain favorable places they exist in 
great variety and abundance. Fishes of types not unlike 
those found at the surface, swim about in the depths of the 
sea ; starfishes, crabs, and shrimp, crawl over the bottom 
ooze ; shells not unlike those which we find along the sea- 
shore, live on the bottom or burrow into it ; and some forms 
exist attached to solid parts of the bottom, while others are 
permanently attached by means of root-like extensions of 
the body, which ramify through the mud. Among the 
animals of the ocean bottom, are found certain types that 
in an earlier stage in the history of the earth were quite 
abundant, but which do not now exist elsewhere, — as if 
they had retreated to this place as an asylum where changes 
and struggles are practically absent. 

As in other portions of the ocean, temperature is the main 
cause for variations in the kind of animals dwelling on the 
sea floor. A change of one or two degrees causes an almost 
absolute change in the faunas. This is in large part because 
of the unvarying conditions of the ocean bottom. There is 
no effect of day and night, nor of season ; but year after year, 



GENERAL CHARACTERISTICS OF THE OCEAN. 171 

and age after age, the conditions of temperature remain the 
same. Therefore animals which have become accustomed 
to a practically permanent condition of 35°, will find a de- 
crease in temperature to 33° so great that they cannot 
survive the change. 

Since these deep-sea animals live amid conditions of un- 
varying temperature, there is naturally a very great decrease 
in vitality as the temperature decreases. And with perma- 
nent temperature conditions of 32° (or as in some cases even 
of 31°), the possibility for the existence of life becomes very 
much decreased. Therefore in the coldest zones of the 
ocean, the abundance of animals is not great. 

Another feature upon which the life of the ocean bottom 
depends, is that of food supply. So far as we are able to 
judge, the animals of the ocean bottom exist partly upon 
one another, but mainly and ultimately upon a supply of 
food that rains down upon them from above. The death 
of the animals of the surface constantly supplies the bot- 
tom creatures with the necessary food. As it sinks, each 
tiny Globigerina serves as a morsel for some animal of the 
ocean bottom ; and the lack of abundance of this kind of 
food supply, seems to place a limitation upon the excessive 
development of animals on the ocean floor. This is probably 
one of the reasons why the variety and abundance of the 
bottom animals is not greater. There is not food enough 
for many more to exist. 

The animals of the ocean depend upon a supply of oxygen 
for breathing ; and this is as true of the animals of the ocean 
bottom as it is of those at the surface. It is not difficult to 
understand how the creatures that dwell in the surface 
waters are able to obtain their supply of oxygen, for the 
surface of the ocean is in constant contact with the great 
body of air. In the case of the animals of the ocean bottom, 



172 PHYSICAL GEOGRAPHY. 

this is far from being true ; and yet they are constantly 
supplying to the water a certain amount of carbonic acid gas 
which in the course of time would tend to so vitiate the 
water that life could not exist. 

This is one of the strongest arguments in favor of a cir- 
culation of the waters along the bottom of the ocean, from 
polar to tropical regions. There must be some supply of 
oxygen furnished to these deep-sea animals, otherwise they 
could not exist ; and there is no other supply known than 
that which may be brought by this great oceanic circulation. 

Since everything points to the conclusion that this series 
of ocean movements along the bottom is very slow, it is not 
unlikely that another limitation to the spread of deep-sea 
animals, is the lack of abundant oxygen. For if there is not 
much supplied to the water, there cannot be much taken out. 
Therefore the existence of life on the ocean bottom, appears 
to depend upon several conditions which are more or less 
important ; one of these is temperature, another is food 
supply, and a third is a supply of oxygen. 



REFERENCE BOOKS. 

Williams. — The Geography of the Oceans. Philip & Son, London, 1881. 
16mo. New edition in the press. (An accumulation of fact and purely 
descriptive matter.) 

Shaler. — Sea and Land. Scribner, New York, 1894. 8vo. $2.50. (Much 
information and discussion, particularly with relation to the coast line.) 

Thomson. — The Depths of the Sea. Macmillan & Co., New York, 1873. 
8vo. $7.50. (A general discussion of the life and conditions of the ocean 
depths.) 

Thomson. — The Atlantic. McDonough, Albany, N.Y. 8vo. Vol. T. and 
II., $3.00. [Published originally by Harper Bros.] (Very full account 
of the conditions existing on the ocean bottom, as revealed by the explora- 
tions of the British ship Challenger.) 



GENERAL CHARACTERISTICS OF THE OCEAN. 173 

Keports on the voyage of the Challenger. — Narrative. Vol. I., Parts I. 
and II. Eyre and Spottiswoode, London, 1885. 4to. £5 16s. 6d. Pub- 
lished for the British government. See also Summary, Vol. I. Price 80s. 
(The best and latest account of the history of the deep-sea exploration. 
Contains several excellent charts of the ocean bottom.) 

Agassiz. — Three Cruises op the Blake. Houghton, Mifflin & Co., 
Boston, 1888. 8vo. Vol. I. and II., $8.00. (The most recent and accurate 
description of the depths of the Atlantic, particularly of the Gulf and 
Caribbean region.) 

Wild. — Thalassa. Marcus Ward & Co., London, 1877. 8vo. 12s. (Much 
on depth, temperature, and currents. ) 

Thoulet. — Oceanographie (Statique). Baudoin, Paris, 1890. 8vo. 10 fr. 

(Much of importance on the physical questions relating to the ocean.) 
Sigsbee. — Deep-sea Sounding and Dredging. United States Coast Survey, 

Washington, 1880. (A splendidly illustrated description of the methods 

employed in deep-sea exploration.) 

Holder. — Living Lights. Scribner, New York, 1887. 8vo. $1.75. (A 
popular description of phosphorescent animals on the land and in the sea. ) 

Murray and Renard. — Volume on Deep-sea Deposits, in the Challenger 
Reports. Eyre & Spottiswoode, London, 1891. 4to. 42s. (A very 
complete discussion of deep-sea deposits. Beautifully illustrated. ) 

Moseley. — Notes by a Naturalist. Murray, London, 1892. 8vo. 9s. 
(Narrative based upon the voyage of the Challenger, and containing much 
on animal distribution and peculiarities.) 

The immense mass of information on this subject accumulated by the 
Challenger is published in an extensive series of over thirty quarto volumes. 
The set is very expensive ; but many of the points of most general interest 
are found in the two volumes of Narrative and the Summary referred 
to above. 

The Annual Reports of the U. S. Fish Commission also contain much on 
deep-sea exploration ; but it is scattered, and mainly found in the earlier 
volumes, which are now difficult to obtain free of cost. 



CHAPTER X. 

OCEAN WAVES AND CURRENTS. 

Wind Waves. 1 — As a result of friction between wind and 
water, the ocean surface is readily started in motion in a 




Fig. 80. 
Ocean waves. Copyrighted, 1871, by Proctor Bros., Gloucester, Mass. 

series of wave-like risings and fallings. Normally these 
wind waves are swells, with alternate ridge-like troughs 
and crests ; but where broken by violent winds, they may 

1 For discussion of the effect of waves on the coast, see Chapter XVIII- 

174 



OCEAN WAVES AND CUBBENTS. 



175 



be cut into a series of chops or angular crests (Fig. 80). 
The water movement consists of oscillatory risings and fall- 
ings of water particles, while the waveform passes across the 
water in the direction toward which the wind is blowing. 
As the wave passes on, a floating object rises and falls as the 
troughs and crests of the waves pass over the surface, show- 
ing that the water itself is not in horizontal movement. 
In reality, the friction of the air does drive some of the sur- 




Fig. 81. 
Breakers on the coast. 



face water along, and therefore if a body could float entirely 
submerged in water, so as to be out of the direct influence 
of the wind, as each wave passed on, it would continue to 
rise and fall, but it would also move a short distance in the 
direction toward which the wave was moving. 

When a wave approaches the shore, its form and behavior 
are greatly changed. The rising and falling particles of 
water encounter the bottom, the top of the wave combs over, 



176 PHYSICAL GEOGRAPHY. 

and it dashes upon the coast in the form of a breaker 
(Fig. 81). The wave is such a shallow movement in the 
water that it is readily destroyed upon reaching ah irregular 
coast. Thus in harbors or bays, the violent ocean waves 
lose their force, largely because of friction upon the shores 
and bottom. 

A very slight breeze will cause a series of wave-like move- 
ments or ripples ; but as the wind continues, and its force 
increases, the water surface may be thrown into a series of 
great undulations. The water is so mobile that these wave 
movements are transmitted for great distances, and they 
often extend far beyond the place of origin. One may see 
this illustrated upon the surface of almost any lake over 
which a steamer is passing. The series of waves started by 
the movement of the steamer through the water, extend out- 
ward for miles before losing their form. Upon the ocean it 
is not uncommon to find great swells or rollers, although the 
sky is clear, the air calm, and the water glassy, — their origin 
generally being some distant storm. 

During almost all times of day, even when the air is quiet, 
the waves beat upon exposed coasts. When the winds are 
severe, waves often rise to unusual heights and beat against 
the coast with terrific violence. They dash against the 
exposed highlands, sending spray into the air, often to the 
height of two or three hundred feet ; and at these unusual 
times, great boulders may be wrested from the rocky shore 
and hurled above the line of the ordinary ocean surface. 
In some cases, in times of unusual storms, lighthouses h°^ r e 
been washed away. Usually the effect of the waves is con- 
fined to that part of the coast which is within a few feet of 
high-tide mark. But during these unusual storms, the action 
of the wind waves may be extended a number of feet above 
this point, reaching places which for many years had been 



OCEAN WAVES AND CUEBENTS. 



177 



considered safe from wave attack. During a storm, a few- 
years ago, many summer cottages on the sandy coast of New 
Jersey were attacked and destroyed by the waves, and a 
railroad that crosses the beach was torn down (Fig. 82). 

These effects of waves attract our attention because they 
are unusual ; but the every-day action of the wind waves is 
also of great importance. They are constantly battering 




Fro. 82. 
Effect of storm waves on the New Jersey coast. 

against the coast and tending to wear it away, while the 
wind-formed currents and the undertow are important aids 
in the removal of the loose materials thus wrested from the 
shore. In many places, as for instance in Boston Harbor, it 
has been found necessary to build sea walls in order to save 
from destruction some of the exposed islands which are com- 
posed of unconsolidated gravel. 



178 PHYSICAL GEOGRAPHY. 

If we watch the rushing of the waves against exposed 
coasts, or the breaking of the rollers upon the sloping 
beaches, we are able to form some conception of the vast 
amount of destructive work that these oceanic agents may 
do in the course of long periods of time. With every rush 
of the water upon the beach, pebbles and sand are dragged 
backward and forward ; and this constant friction of one 
particle upon another, in the course of time will cause even 
the hardest rocks to wear away. In the course of a few 
years fragments of brick or glass become rounded, so that 
they resemble the form of the true beach pebbles ; and in a 
year or two a brick may be reduced to a pebble only a small 
fraction of the size of the original. 

Earthquake Waves. — When an earthquake shock disturbs 
the waters of the ocean, a great wave is formed, which 
extends from the bottom of the sea to the surface, and 
which is therefore much more profound in its effect than the 
shallow wind waves. In the mid-ocean these earthquake 
waves may not be perceptible ; but as they reach shallow 
coasts, they may become noticeable as their elevation is 
increased in the shallowing water. Upon reaching coasts 
not far from the point of origin, they may have a height of 
from 50 to 100 feet, which gives them the power of rushing 
upon the shore to a much greater distance than ordinary 
waves are capable of reaching. 

During some earthquake shocks, the water wave has 
extended over low coasts and destroyed scores of thousands 
of lives. Fortunately this form of ocean disturbance is 
rare, and it is a type of wave which is not common in the 
Atlantic Ocean. Along the west coast of South America, 
and on the Asiatic coast, where earthquakes and volcanic 
eruptions are frequent, the earthquake wave assumes very 
great importance. It travels at a rate of from three to four 



OCEAN WAVES AND CURRENTS. 179 

hundred miles an hour, and may extend for a distance of six 
or seven thousand miles from the place of origin; but at 
such great distances it has so lost its force that it produces 
no destructive effect. 

Among the important effects of these rare waves is the 
destruction of life in the ocean. An explosion of dynamite 
in water will kill the fishes that are exposed to the shock ; 
and near its source, the earthquake wave tends to cause the 
same kind of destruction. 

Storm Waves. — When the great whirling storms of cyclonic 
origin (the hurricanes and temperate latitude cyclones de- 
scribed in Chapter V.) pass over the ocean, the spirally 
inblowing winds tend to heap up the water near the center 
of the storm. In the center the air pressure is less than on 
the margins, and this also causes the water near the center 
of the storm to rise. Therefore during these storms there 
are two tendencies to the production of unusually high water. 
When the storm centers pass along the coast, the ocean sur- 
face is raised to a height often as great as six or eight feet 
above the average ; and if violent wind waves accompany 
this high state of water, their destructiveness along the shore 
becomes greatly increased. 

Any strong prevailing wind blowing upon the coast, tends 
to raise the water to an unusual height. During the passage 
of waterspouts over a portion of the ocean, there is raised 
a cone-shaped wave, a few yards across the base, which, on a 
small scale, resembles that caused by the passage of hurricanes. 

Ocean Surface Temperatures. — Latitude is the most impor- 
tant cause for differences in atmospheric temperature, and 
the same is true for the ocean. Near the equator the oceanic 
waters are warmed, while near the poles their temperature 
remains approximately at the freezing-point throughout the 
year. There are all gradations between these two extremes 



180 



PHYSICAL GEOGRAPHY. 



Fathoms i SO _Fahr, 

200 
300 



(Plates 15 and 27). As in the case of the atmosphere, this 
regularity of distribution is interfered with by outside 
causes, mainly the influence of land, and air and water 
movements (Plates 15 and 27). The influence of the ocean 
currents is shown in both of these maps ; and they also show 
the greater regularity of the ocean surface 
isotherms in the southern hemisphere, where 
there is little land. 

Near the coast the temperatures of the 
ocean surface are subjected to very marked 
variations. This is particularly true in the 
temperate zones, where the difference be- 
tween summer and winter temperatures is 
Yerj great. Thus, on the New England 
coast, the water in summer is warm enough 
for purposes of bathing, while in winter it is 
not uncommonly frozen in the shallow har- 
bors. Even at a distance of a number of 
miles from the shore, this variation from 
summer to winter is quite marked ; but in 
the mid-ocean, and in the tropical and arctic 
zones, the summer and winter temperatures 
are very nearly the same. 

In the ocean there is a vertical change in 
temperature. Since water warms very slowly, 
the effect of the sun extends only to a dis- 
tance of a few score of feet, even in the trop- 
ics ; and below this, the temperature throughout the year is 
practically uniform, while it rapidly descends until the cold 
waters of the great ocean depths are encountered (Fig. 
83). Because radiation from a water surface is a slow 
process, the temperature of the water does not become 
rapidly lowered during the night. Therefore there is very 



Via. So. 

Diagram to show 
the normal de- 
scent of temper- 
ature in a col- 
umn of water 
in the ocean at 
the equator. 




Plates 15. 



182 PHYSICAL GEOGRAPHY. 

little reason for decided changes in temperature, either 
between the day and night, or between the seasons. 

Ocean Currents : Planetary Circulation. — As a result of 
differences in temperature between polar and tropical re- 
gions, the air is engaged in a series of great movements. 
There are many reasons for believing that a similar circu- 
lation exists in the ocean. The fact of the difference in 
temperature suggests the probability of such a circulation, 
which would consist of a rising of the water under the 
equator, a surface outflow from equatorial to polar regions, 
and then a downsinking to the bottom, from which there 
would be a return to the equatorial regions along the ocean 
bottom. 

That this theoretical circulation actually exists, is sug- 
gested by the fact that the bottom of the sea is inhabited by 
large numbers of animals. If some such circulation as this 
did not exist, it would be difficult to account for the supply 
of oxygen which these creatures need for their existence 
(pages 171 and 172). 

Such planetary circulation seems also demanded by the 
temperature conditions of the ocean bottom. Unless there 
has been a downsinking and passage of Arctic waters over the 
bottom of the ocean, we cannot explain the fact that temper- 
atures as low as the freezing-point of fresh water exist over 
great areas in the depths of the sea. 

A circulation is also suggested by the peculiar distribution 
of temperature on the ocean bottom. It was stated on 
page 162 that in some deep parts of the sea, the tempera- 
tures are higher than in other portions whose depth is not so 
great, the apparent explanation being a barrier which inter- 
feres with the passage of the slowly moving water over the 
ocean bottom. Then also, under the equator, a temperature 
of 41° is encountered at a depth of 250 fathoms, while in the 




Face page 183. 










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- 1 






Jt.D.Stno,,.ll.Y. 


Warm, Currents 


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Cold Currents 


ffel 













OCEAN WAVES AND CURRENT 8. 183 

northern hemisphere this temperature is reached at a 
depth of about 600 fathoms, and in the southern hemi- 
sphere at a depth of about 400 fathoms. This indicates 
that under the equator the cold water of the ocean bottom 
is rising. 

The System of Ocean Currents. — (Plate 16.) We know 
little concerning the circulation of the water on the sea 
floor; but at the surface there are certain very distinct 
movements, to which the name ocean currents is given. In 
the Atlantic there is a drift of surface water toward the 
equatorial portion of South America. This slowly moving 
surface water divides against the triangular coast of South 
America, one portion passing southward, the other and 
larger part moving northward, still as a slowly moving 
drift. 

A considerable part of the north-moving drift passes into 
the North Atlantic outside of the West Indies, and this may 
be called the North Atlantic Drift. The portion which enters 
the Caribbean, passes through the Straits of Yucatan into the 
Gulf of Mexico, where a part of it circles around, and finally 
emerges past Key West at the southern end of Florida. 
Some of the water passes northward between the West 
Indies. Therefore, by the time it has reached the latitude 
of the Carolinas the warm current of the North Atlantic is 
composed of several parts. 

The portion which emerges from the Gulf of Mexico 
between Cuba and Florida, is known as the Gulf Stream ; 
and this passes northward along the coast as a very percep- 
tible current, on the seaward side of which is a portion of 
the Atlantic drift, which did not enter the Gulf of Mexico. 
At about the latitude of Cape Hatteras, the Gulf Stream is 
turned to the right as a result of the influence of the earth's 
rotation, and it then passes out into the Atlantic until the 



184 PHYSICAL GEOGRAPHY. 

European shore is neared. 1 A branch extends northward 
into the Arctic, while a part returns as a surface current 
along the coast of Spain and Africa, there joining the equa- 
torial drift. The water thus eddies around in the North 
Atlantic, moving northward, then eastward, southward, and 
southwestward, thus establishing a complete whirl. Another 
important current in the North Atlantic is the cold Labra- 
dor current (see p. 189). 

In the South Atlantic a similar whirl of water is caused ; 
but this is distinctly less pronounced than that of the North 
Atlantic, and there is no current so marked as the Gulf 
Stream. Cold water from the Antarctic extends northward 
into the South Atlantic. 

In the North Pacific, a circulation is established which 
very closely resembles that of the North Atlantic. A broad 
equatorial drift passes westward toward the Asiatic coast, 
then becoming in part a north-moving current, it proceeds 
as a very distinct stream, in many respects resembling the- 
Gulf Stream. This is known under the name of the Kurc 
Siwo, or better as the Japanese current. It passes north- 
ward, is turned to the right, then moves southeastward, 
bathing the western coast of the United States, then curv- 
ing to the southwest, it joins the equatorial drift. Owing 
to the fact that land practically excludes the Arctic 
waters from the North Pacific, there is no distinct Arctic 
current in this ocean, nor is the Japanese current able to 
extend a large branch into the Arctic. Still a small current 
of cold water does pass through Bering's Straits into the 
North Pacific. 

In the South Pacific and Indian oceans, distinct whirls of 

1 Numerous observations on the movements of wrecks and floating bottles 
have given us much information concerning this current. One set of obser- 
vations upon a floating wreck is shown on Plate 16. 



OCEAN WAVES AND CURRENTS. 185 

water are produced, which more nearly resemble the whirl of 
the South Atlantic than those of the northern oceans, but 
which nevertheless are better developed than the South 
Atlantic system of currents. Cold currents from the 
Antarctic extend into both of these oceans. 

Thus we find a great series of whirls in the oceans, one on 
each side of the equator in each ocean, the water passing 
poleward as surface currents and in part returning to com- 
plete the whirl. The north-moving system is better devel- 
oped than the return south-moving currents, and the system 
of ocean currents is much better developed in the northern 
than in the southern oceans. Particularly is this true of 
the Atlantic. In any explanation of oceanic circulation 
these facts must be accounted for. 

Cause of Ocean Currents. — Although there is evidence 
that a planetary circulation exists in the ocean, there are 
many reasons for doubting whether this cause is sufficient to 
explain the system of surface currents which is so well devel- 
oped. The difference in temperature does not seem suffi- 
cient to account for the great oceanic whirls. While the al- 
most constant cold of the Arctic and Antarctic oceans causes 
a continual descent of water, it cannot be said that the heat 
at the equator is sufficient to so expand the water as to 
cause surface currents to start here. 

The comparison has been made between the ocean circu- 
lation and that of the air ; but this is only partly warranted, 
for the air is warmed from below by contact with the earth, 
and when warmed this lower air must rise. In the ocean, 
the heat of the sun is practically confined to the immediate 
surface ; and this relatively thin layer is not warmed to a 
very high degree, so that its expansion would not seem to 
be sufficient to cause a flowing away. If the sun's heat 
penetrated to the ocean bottom, the warming of the lower 



186 PHYSICAL GEOGRAPHY. 

layers of water would cause sufficient expansion to necessi- 
tate their rise to the surface ; but this is not the case. 

If temperature differences account for ocean currents, the 
fact of the greater development of the system in the north- 
ern oceans would be difficult to explain. The Antarctic is 
practically open to both Atlantic and Pacific, and in that 
hemisphere there is an excellent opportunity for an exchange 
of polar and tropical waters. But the Arctic is almost com- 
pletely shut off from the Pacific, and is only open to the 
Atlantic through narrow and rather shallow channels. 
Therefore, in the hemisphere where the least favorable con- 
ditions for an exchange of water exist, we have the best 
developed currents. In the North Pacific there seems abso- 
lutely no chance for the general passage of cold northern 
waters along the bottom to the equator. 

This and other reasons, such for instance as the presence 
of cold surface currents returning from the Arctic, cause 
great doubt as to the validity of the temperature theory 
which has been held by many physicists. It seems that we 
are forced by these arguments to return to the theory which 
was proposed by Benjamin Franklin, who pointed out the 
fact that nearly permanent winds are blowing toward the 
equator throughout the year. These trade winds necessarily 
drive large quantities of surface water before them, just as 
the winds along the coast will cause the surface water to 
drift before them. 

Thus the water is being heaped up in equatorial regions, 
and this seems sufficient to account for the great whirls ; and 
there are many facts tending toward the conclusion that 
winds are the prime cause for these currents. Among other 
things, the whirls are best developed in the northern oceans. 
The belt of calms, which separates the two systems of trade 
winds, is mostly north of the equator, and the northern belt 



OCEAN WAVES AND CUBBENTS. 187 

of trade winds does not extend south of the equator during 
the northern winter, while the southern belt does extend 
north of the equator during the northern summer (Plates 10 
and 11). Therefore there is a greater drift of water in the 
northern hemisphere than in the southern. Probably the 
differences in temperature aid in this circulation ; but to 
this cause we must assign a secondary importance. 

The course of the various currents is in part determined 
by the outlines of the continents. If there were no conti- 
nents, the effect of the trade winds would be to produce a 
surface drift, which would tend to pass around the earth, 
approximately in the belt of calms. The north and south 
extension of land in the form of continents, interferes with 
this circulation, and causes the moving water to pass north- 
ward or southward. After this deflection, there is a con- 
tinued tendency for the currents to turn, under the influence 
of the earth's rotation, to the right in the northern, and to 
the left in the southern hemisphere. These two facts of 
continental interference, and deflective effect of the earth's 
rotation, are mainly responsible for the paths pursued by the 
currents, and for the great system of whirls. The cold sur- 
face currents which come from the Arctic and Antarctic, are 
probably a partial return of the warm water that drifts into 
these zones. 

The Gulf Stream. — This, which is the best-known of ocean 
currents, is of so much interest to us, and so well illustrates 
some minor phenomena of ocean currents, that it is well 
to examine it in a little more detail than has been done 
(Plate 17). The Gulf Stream proper is that portion of the 
equatorial drift which has passed through the Caribbean 
and the Gulf of Mexico. During its passage through these 
warm gulfs, its temperature has been increased so that it 
emerges into the Atlantic as a very warm current. It is 



188 



PHYSICAL GEOGRAPHY. 



one of the most rapidly moving of ocean currents, and its 
rapidity depends upon the peculiar effect of irregularities in 



CHART OF THE 

aTJXF STREAM 

SHOWING ITS AXIS AND LIMITS 



Boston c^ cifpe'.Cod--'"" 

NANTUCKET I. 



Washington 




P.LAXE H. 

the continental outline. Passing into the Gulf of Mexico 
without difficulty, it finds itself partially enclosed. The one 



OCEAN WAVES AND CUBBENTS. 189 

place of easy escape is in the narrow passage between Key 
West and Cuba. In a measure, it is concentrated here, in 
a manner somewhat analogous to the concentration of water 
in the nozzle of a hose. 

When it passes through the channel at the end of the 
Yucatan peninsula, its velocity is only about ^ of a mile 
an hour, and its width is about 90 miles, while its depth 
is approximately 1000 fathoms. When it emerges past 
Key West, its velocity is from four to five miles an hour, its 
width only 50 miles, and its depth about 350 fathoms. If it 
were not for this concentration, the Gulf Stream would not be 
such an important factor in the North Atlantic. Soon after 
passing through this narrow channel its velocity decreases ; 
and by the time it has reached the Banks of Newfoundland, 
its rate of movement is less than half that which it had on 
the Florida coast. It has been estimated that every day, the 
Gulf Stream carries past Florida the enormous amount of 
136,000,000,000,000 tons of water. 

The Labrador Current. — The Labrador current comes from 
the Arctic between Greenland and Labrador, passing down 
the coast of Nova Scotia and New England, and keeping 
close to the coast, because of the influence of the earth's 
rotation, which tends to make it curve to the right. It 
remains as a surface current until Massachusetts Bay is 
reached, where it sinks to the bottom, owing to the fact that 
it has a lower temperature, and therefore greater density, 
than the surrounding water. But its influence is felt upon 
the continental shelf nearly down as far as Cape Hatteras. 

Effects of Ocean Currents. — The most striking effect of 
currents is upon the temperature. If it were not for the 
existence of this oceanic circulation, it is probable that 
a large part of the now habitable earth would be ren- 
dered unfit for habitation. Much of the heat received in 



190 PHYSICAL GEOGRAPHY. 

equatorial regions would remain there, while the cold of 
high latitudes would increase, and the temperature in 
these regions would be reduced to very low degrees. The 
equatorial regions would be much hotter than at present, 
while the high temperate and arctic belts would be colder. 
The immense influence of these currents is shown by the 
fact that in the latitude of Labrador, — a bleak, inhospitable 
land, — there are powerful and well-populated countries on 
the other side of the ocean. In the one case, cold Arctic 
currents flow along the coast; in the other, the climate is 
tempered by the warm ocean current. 

Ocean streams carry vastly more heat than air currents 
are capable of doing. Croll has estimated that the Gulf 
Stream alone carries as much heat as falls upon a surface 
of 1,560,935 square miles at the equator. He says that this 
stream carries from tropical regions, nearly one-half as much 
heat as is received directly from the sun in the entire Arctic. 

The temperature of the Pacific coast of the United States 
is greatly moderated by the warm Japanese current, which 
carries into the North Pacific a large store of heat ; and, 
both on this coast and on the western coast of Europe, the 
warm bodies of water which are off-shore, not only supply 
quantities of heat, but they furnish to the land much moist- 
ure which is condensed in the form of rain. 

Franklin's attention was called to the Gulf Stream by 
reason of the fact that sailing vessels made their voyage 
from the colonies to the mother country in a shorter time 
than on the return. Therefore, in some cases, currents in 
the ocean are an aid to navigation (see Plate 16, showing 
the drifting of a wreck in this current). Where a cold 
and warm current are side by side, as is the case near 
Newfoundland, fogs are abundant, and this interferes with 
navigation. 



OCEAN WAVES AND CURRENTS. 191 

Since the currents temper the ocean waters (see Plate 15), 
they tend to modify the conditions upon which the spread 
of marine animals depends. This influence is particularly 
noticeable among the coral reefs. The warm tropical cur- 
rents carry large quantities of food and of clear water to 
the banks upon which corals are developing ; and it may be 
said that in this indirect way, ocean currents are an impor- 
tant cause for coral reefs. Thus, where the Gulf Stream 
bathes the coast of Florida, reefs and coral keys are pro- 
duced ; and even as far north as the Bermudas, coral life 
is possible because of the presence of the warm tropical 
current. 

REFERENCE BOOKS. 

In many of the books referred to at the end of the last chapter, there is 

something on oceanic movements. See particularly Agassiz, Wild, and 

Thoulet. 

For a very complete discussion of ocean currents, see Croll's " Climate 

and Time," referred to at the end of Chapter VII. 

Maury. — The Physical Geography of the Sea. (There are many edi- 
tions of this book, most of them, and the best, being out of print and 
obtainable only in the second-hand condition.) 

Pillsbury. — The Gulf Stream, " U. S. Coast Survey Annual Keport for 
1890," Appendix 10. Washington, 1891. Issued by the survey in sepa- 
rate form. (The most complete discussion of the Gulf Stream which has 
been printed.) 

Berghaus Atlas, volume on Hydrography. Justus Perthes, Gotha, 
Germany, 1891. 15m. (Contains numerous excellent charts of currents, 
ocean temperatures, etc.) 



CHAPTER XI. 

TIDES. 

Nature of the Tidal Wave. — Each day the ocean surface 
is disturbed by two waves which pass about the earth with 
great rapidity (fully 500 miles an hour in the Atlantic), and 
affect the entire ocean, from surface to bottom. The actual 
height of the tidal wave is very slight, and sailing vessels in 
the mid-ocean are never aware of its existence. When it 
approaches the shore, the wave is subjected to a variety of 
complex changes, which make it an important feature of the 
ocean. On the coast, the water gradually rises or flows, and 
as gradually falls or ebbs; and this is repeated, with marked 
regularity, approximately twice each day. 

Cause of Tides. — In origin, the tide is directly associated 
with the effect of the moon and sun upon the earth. All 
bodies in space are engaged in a mutual attraction which we 
know as gravitation ; and the effect of this gravitative at- 
traction is proportional to the product of the masses, and 
inversely proportional to the square of the distance. Every 
member of the solar system is exerting an attraction upon 
the earth. Since the attraction varies with the mass, such 
a large body as the sun would produce a great effect if its 
distance were not so great. The moon, although relatively 
small, is so near that its influence upon the earth is much 
greater than that of the larger and more distant sun. 

Leaving the sun out of the question for a time, let us see 
what effect the attraction of the moon will have upon the 

192 



TIDES. 193 

earth. If the earth were all liquid, the attractive action of 
the moon would tend to destroy the sphere and change it to 
an ellipse. The ellipse would project toward the moon, for 
that portion of the earth's surface which was nearest the 
moon would be most attracted. Since the rotation of the 
earth causes the moon to appear to pass through the heav- 
ens, this ellipse would constantly change in position, with 
its axis always pointing toward the moon. Therefore the 
liquid sphere would be thrown into a series of waves, one 
crest being beneath the moon, while the other crest was on 
the opposite side of the earth farthest from the moon. 

This is approximately what happens in the liquid ocean. 
The surface of this partial liquid covering is disturbed by 
the gravitative attraction of the moon, and a wave is thus 
produced beneath it, while one is also formed on the opposite 
side of the earth. As the moon appears to pass around the 
earth, these two waves also move. They are much disturbed 
by the irregularity of the continents, and their movements 
are rendered very complex as a result of this influence. 
They are not able to remain directly beneath the moon, but 
lag behind and follow it, instead of passing around the earth 
with it. 

In a similar manner the sun causes two waves ; and these 
combine with those produced by the moon to produce the 
tidal wave. Since the movements of the sun, the moon, and 
the earth are very irregular, there are many complexities 
introduced into the tidal movement. In the latter part of 
the chapter some of these irregularities are considered, and 
their cause pointed out. 

Effect of the Land. — The tide waves tend to pass about 
the earth from east to west, following the direction of the 
path of the moon through the heavens. This west-moving 
tide wave is much better developed in the great expanse of 



194 PHYSICAL GEOGRAPHY. 

water in the southern hemisphere, than in the northern. 
The continents interfere with the movement of the wave, 
and in some cases successfully check it. When the wave 
enters the Atlantic, its direction is changed from west to 
north, and soon the wave is so changed that it advances 
more rapidly in the middle part of the ocean than on the 
margins (Plate 18). This is due to the effect of the shallow 
waters near the continents. The wave is retarded near the 
shore and advances more rapidly in the central portion of 
the ocean. As a result of this, the crest of a wave may have 
reached the latitude of Newfoundland at the same time that 
the margins are affecting the coast of northern Africa and 
the West Indies. 

In a similar way this effect of friction is also shown in the 
bays and larger estuaries. Thus as the wave passes up the 
Bay of Fundy, friction with the shore causes it to be re- 
tarded, while in the central part of the bay the wave advance? 
more rapidly. 

Nowhere can this effect of the land be better illustrated 
than in the vicinity of the British Isles (Plate 19). The 
wave passes up the Atlantic, and without serious inter- 
ference moves to the northern extremity of Ireland and Scot- 
land, while the same wave has advanced to the southern 
coast of England, and begun to pass into the English Chan- 
nel. The shallowness of the water in this region prevents 
the rapid movement of the wave ; and by the time it has 
passed through the English Channel, the part that went out- 
side of the British Isles has gone entirely around the islands, 
and entered the North Sea, where the two parts of the same 
wave meet (Plate 19). 

On the American coast a similar influence of the land is 
noticed in the approaches to New York Harbor. The tida] 
wave passes readily up the bay toward New York, while the 







Face page 194.. 



Diagrammatic representation of the advanc 







ives. Figures refer to noon and midnight. 




Plate 19. 

Magrammatic representation of the tidal wave near the British Isles. Figures 
refer to hours of the day. 



196 



PHYSICAL GEOGRAPHY. 



same wave goes around the eastern end of Long Island into 
Long Island Sound (Fig. 84). Here its rate of motion is 

retarded, the dis- 
tance traveled is 
greater, and at 
Hell Gate Chan- 
nel the two parts 
of the same wave 
arrive at entirely 
different times 
(Fig. 85). This 
is one of the rea- 
sons for the vio- 
lent currents at 
Hell Gate, 
coastal irregularities upon 




Fig. 84. 

Diagram to show path pursued hy the tides on the two 
sides of Hell Gate. Figures represent height of tides 
at different places. 



Aside from this influence 
the time of approach of 
the tidal wave, these pe- 
culiarities also influence 
the height to which the 
tide rises. The normal 
tidal rise, as observed 
in mid-ocean and on ex- 
posed coasts, is only one 
or two feet. Along the 
eastern coast of America, 
we find the tide rising 
in one place only two or 
three feet, in other places 
10 or 20 feet, and in the 
Bay of Fundy often 50 or 
60 feet. These irres'u- 



of 



HOURS AFTER TRANSIT 
III IV V VI VII VIII IX X XI 




1 1 


1 




















































































































































/* 






















































/P 


























'£ 








* 


























h 


















































4° 




























*v 







































































































Fig. 85. 
Diagram to show time of arrival and height 
reached by the tides on the two sides of 
Hell Gate. 



larities are due to the influence of the coastal outline. 



TIDES. 197 

There are two ways in which, the tide may be almost 
entirely destroyed. It is a familiar fact, that if two waves 
meet trough to crest, they extinguish one another. It is 
believed that the two tidal waves which meet in the North 
Sea (Plate 19), actually come together in this way. 

When the tide enters a large body of water through a 
narrow inlet, the tidal rise is almost entirely destroyed, as is 
very well illustrated in the Mediterranean. Outside of the 
Straits of Gibraltar, on the coast of Spain, the height of the 
tide is from five to six feet. The wave enters the Mediter- 
ranean through this narrow inlet, then expands, and con- 
sequently loses in height, until almost no tide is left. In 
portions of the Mediterranean there are slight tides, but 
these appear to depend in part upon another cause. 

The opposite effect of increase in height of the tide is by far 
the most common influence of coast irregularities. When, 
instead of entering a large body of water through a narrow 
inlet, the tidal wave passes into a narrowing bay through a 
broad mouth, the effect of the converging shores is to pile 
up the wave, and therefore to increase the height of the tide. 
This is the cause for the very high tides of the Bay of Fundy, 
and many other V-shaped bays and estuaries. It is well 
illustrated in Massachusetts Bay, where the rise of the tide 
is between 8 and 12 feet. 

As a result of the influence of coast irregularities, some 
peculiar tidal effects are produced. In two neighboring, and 
possibly connected bays, the height of the tide may be quite 
different. This is the case in Vineyard Sound and Buzzard's 
Bay, on the south coast of Massachusetts, where, in the lat- 
ter, the tide rises one or two feet higher than in Vineyard 
Sound, which is open on both ends. In the channels which 
connect these two bays, violent currents are produced; 
and this whole region, between the Elizabeth Islands and 



198 PHYSICAL OEOGBAPHT. 

Nantucket, is one of relatively rapid tidal currents. The 
rapid currents in the straits between two such bodies of 
water, may be called tidal races. 

A tidal race is produced at Hell Gate, near New York 
City, mainly because the tide rises higher in Long Island 
Sound than it does in the bay of New York Harbor (Figs. 84 
and 85). The very rapid currents in this shallow strait, are 
in part due to this cause, and in part to the fact that the 
time of high tide is different on the two sides of Hell Gate. 
Similar tidal races occur on many parts of the irregular 
northern shore, and at times currents are produced which 
are as violent as rapidly moving streams. In some cases it 
is impossible to row a boat against the current. 

In a rapidly narrowing bay, particularly at the mouth of 
a river, the rising tide is sometimes transformed to a wave, 
which in form resembles the wind wave ; and there is an 
advancing wall of water, instead of the gradual, almost 
imperceptible rising of the ocean surface, which is the normal 
form of the incoming tide. To this peculiar phenomenon 
the name tidal bore is given. This wave is produced in the 
Amazon, the Severn, the Seine, and many other rivers. 

Other Causes for Variation in Tidal Height At any given 

point on the coast, the height of the tide is liable to vary from 
time to time. This variation may be of an irregular nature, 
due to the effect of winds upon the surface of the water. 
Sometimes, when strong winds blow upon the coast, the 
height of the tide may be increased several feet. A mere 
change in the pressure of the air also appears to cause fluc- 
tuations in the surface of the sea ; and upon lakes, these 
causes produce fluctuations in level which are often of quite 
noticeable size. In the Swiss lakes these irregular variations 
in the level of the water are known as seiches, and they are 
also found upon the Great Lakes. 



TIDES. 



199 



The main variations in 
the height of tide depend 
upon astronomical causes. 
Since the tide is the com- 
bination of two waves, one 
produced by the sun, and 
the other by the moon, the 
height of the tide naturally 
varies as the position of 
these bodies in the heavens 
changes. During new 
moon, the sun and moon 
are nearly in the same line, 
and they therefore pull 
approximately along the 
same line, so that the 
unusually high tide then 
produced (known as spring 
tide), is the result of a 
combination of the two 
waves. During full moon, 
the sun and moon are 
again in line, one on either 
side of the earth, and then 
the two waves again tend 
to combine. Therefore 
every month there are two 
sets of rather strong or 
high tides (Fig. 86). 
Between new and full 
moon, — that is, during 
the first and third quar- 
ters, — the sun and moon 



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200 



PHYSICAL GEOGRAPHY. 



are pulling upon the earth at an angle, and then unusually 
weak or low tides, known as neap tides, are produced. 

In the movement of the moon around the earth, it follows 
a path which is quite elliptical. Therefore, since the earth 
is at one of the foci, there is a time during every lunar 
month when the moon is much nearer the earth than when 
it is in the opposite part of its elliptical path. When the 
moon- is nearest to the earth, it is said to be in perigee, and 
when farthest from the earth, in apogee. Since the tide- 
producing force varies greatly with the distance, this differ- 
ence in lunar distance produces a very marked effect upon 



1893 



1894 




Fig. 87. 

Height of the high tide at Eastport, Maine, in 1893 and 1894. Cross indicates 
apogee ; dot, perigee ; shaded circle, new moon ; plain circle, full moon. 



the strength of the tide. Thus it will be seen that when the 
new or full moon comes during perigee, a high range of tides 
will result, because then the moon is both nearer to the 
earth, and its tide is combined with that caused by the sun. 
If new or full moon occurs during apogee, the tides are not so 
strong, because then, although the solar and lunar tides are 
combined, the moon is farther from the earth. When apogee 
occurs at one of the quarters, the tides are unusually low ; 
and when perigee occurs at this time, the effect of opposition 
of sun and moon is partly counterbalanced (Figs. 86 and 87). 
Other movements of the earth, sun, and moon introduce 
complexities in the tidal rise and fall. For instance, in some 



TIDES. 201 

seasons the sun is nearer the earth than at others, and at 
times the moon is more nearly over the equator than at other 
times ;' and all of these variations produce an effect upon 
the tide. One notices in Fig. 86 that the two tides for any 
single day are different in height ; and this difference varies 
in various parts of the month. All of these irregularities 
are capable of explanation, and are well understood ; but it 
will be impossible to give the space for their consideration 
in this book. 

One point is worth special attention, — that the time be- 
tween two high tides is not exactly a half day, because the 
tide wave travels on lunar, and not on solar time. The tide 
rises once every 12 h. 25 m., so that each day the high tide 
is about 50 minutes later than the corresponding tide for 
the previous day. 

Effects of Tides. — Along irregular coasts, where the tide 
rises to a height of several feet, and where tidal currents are 
produced, the influence of the rise and fall of the tide is of 
considerable importance in navigation ; and before sailing, 
many vessels wait until a favorable time of tide. This is 
particularly the case when ships are about to sail from ports 
that are obstructed by bars, which at low tide are so near 
the surface that some ships are unable to pass over them. 
This is the case in many harbors of the world. 

Because they are much less powerful, tidal currents are 
not wearing the coast in the way that wind waves are ; but 
they are doing a certain work in changing the form of the 
coast, mainly as the result of transportation of fragments 
derived from the rocks by the beating of the waves. On 
some coasts, as for instance in the English Channel, and 
near Nantucket, the action of the currents, by the constant 
movement of the sands, is sufficient to cause frequent changes 
in the depth of the water. 



202 



PHYSICAL GEOGRAPHY. 



Where the tide rises in the mouths of rivers or in estua- 
ries, as in Chesapeake and Delaware bays, the rise of the 
tide checks the river water, and causes it to deposit what 
sediment it is carrying, so that this effect is also important 
in modifying the bottom of these bays (Fig. 88). Many 
harbors are being filled by this means, and millions of dollars 
are every year expended in attempting to remove the mud 
and sand deposited by this tidal action. 




Fig. 88. 

Low tide in Basin of Minas, Nova Scotia. An extensive mud flat, submerged at 
high tide. (Copyright, 1890, by S. R. Stoddard, Glens Falls, N.Y.) 

The rise and fall of the tides is a great force in the ocean 
(Fig. 89), constantly acting, and capable of doing a great 
amount of work, which man may sometime find it possible 
to utilize. Already, in some places, the rising and falling 
tides are employed for local purposes of water power. On 
the New England and Canadian coast, the rising tide is 
allowed to freely enter some broad, bay-like expansion of 
the coast, from which it is prevented from escaping by means 



TIDES. 



203 



of gates that automatically close as the tide begins to 
fall. There is then pro- 
duced a rather large pond, f ~~r«t 
several feet above the low- 
water mark ; and from this, 
water may be led upon a 
wheel, and then made to 
serve for mill purposes. 
There are numerous grist 
mills along the coast which 
are run by tide -water 
power. They can be used 
only a few hours every 
day, but it is a very inex- 
pensive power. The intro- 
duction of electricity for so 
many uses, may make it possible to employ this vast force 
much more commonly than has been done. 




Coast of Cape Ann, Mass. To show tidal 
rise and fall. The dark-colored areas are 
covered by the high tide. 



REFERENCE BOOKS. » 

Thomson. — Popular Lectures and Addresses. Vol. III., Lecture on 
the Tides. Macmillan & Co., New York, 1891. 12mo. $2.00. (A par- 
tial statement of the tidal theory). 

See article on tides in Encyclopedia Britannica. 

For data upon time and height of tides, see Tide Tables for the Atlantic 
Coast, U. S. Coast Survey, Washington, D.C. $0.25. Published an- 
nually. There is also a similar set of tables for the Pacific coast. 

1 The subject of tides is difficult to present clearly in non-mathematical 

H terms, and hence the general literature is quite barren upon the subject. By 
far the most that has been written upon the subject, is scattered through the 
proceedings of scientific societies and the magazines. 



Part III. 
TEE LAND, 



CHAPTER XII. 

THE CRUST OF THE EARTH. 

Interior Condition. — Some wells and mines have extended 
to a depth of over a mile from the surface, and in every case 
it is found that the temperature increases as the depth 
becomes greater. While this increase is not regular, on the 
average it is about 1° for every 50 or 60 feet of descent. If 
this increase continues, as it probably does, the temperature 
at the depth of a score of miles, is sufficiently high to melt 
most rocks under the conditions existing at the surface. 
In various parts of the earth, molten rock reaches the surface 
through volcanic vents ; and there are other indications that 
high temperatures exist within the earth. 

Until within a few years, it was believed that beneath 
a crust of comparative thinness, the earth was in a molten 
condition, and that the solid crust, or rind, rested upon this 
liquid. In speaking of the outside of the earth, we still 
use the term crust, although it is no longer believed that 
the interior is molten. Many facts, some astronomical, 
others geological, have caused the abandonment of the theory 
of a molten interior ; and it is now believed, that although 
at depths only a few miles from the surface, the temperature 
is high enough to melt rocks, they are prevented from 
becoming molten by the great pressure of the solid strata of 
the crust. This energy is constantly passing from the interior 
to the surface, where it is radiated into space ; and this 
constant loss of heat causes a loss of bulk through con- 

205 



206 PHYSICAL GEOGRAPHY. 

traction. The cold outside does not shrink ; but as the 
interior loses in size, this crust becomes wrinkled, in a man- 
ner which may be compared with the wrinkling of the skin 
of an apple which is drying. 

Movements of the Crust. — There are many proofs that 
the crust of the earth is in movement. Usually these 
movements are so slow that they can be detected only 
after long intervals of time ; but sometimes rapid changes 
have actually been witnessed. The proofs of these earth 
movements may be said to be of two kinds, historical and 
geological. While the historical proofs may perhaps appear 
to be most conclusive, they are in reality much less impor- 
tant than those of a geological nature. 

In several places the land has been known to move during 
earthquake shocks, and to remain either higher or lower 
than before the shock. As one instance of this, we may refer 
to the earthquake of 1822, during which the whole coast of 
central Chili was raised from three to four feet. In other 
shocks on the same coast, the land has been permanently 
elevated, and there is abundant evidence that the land of 
this coast is now steadily rising. Near Vesuvius, in Italy, 
there are columns of a temple which were built above the 
level of the ocean, and are now above it, but which at one 
time were submerged ; and they therefore register two move- 
ments of the land. On the coast of Sweden, it was believed 
that the land was slowly moving, but so slowly that without 
careful measurements it could not actually be proven. In 
order to thoroughly test the matter, marks were made at 
the water surface, and after a number of years examined, 
when it was found that there were movements over an area 
of 200 miles in extent. North, of Stockholm the rate of 
elevation is as much as two or three feet a century. 

Of geological evidence, perhaps the best is that of fossils, 



THE CBUST OF THE EARTH. 207 

which have attracted attention from the very earliest times. 
Remains of animals that must have dwelt in the sea, are 
found in many of the rocks of all continents, and at all 
elevations, even on the highest mountains. The rocks 
themselves are evidence of elevation, for in many cases 
they are of kinds which we know must have been formed 
in the sea. 

Along the coast lines, in many parts of the earth, beaches 
and other features of the seacoast are found at a distance 
above the present sea level ; and tree trunks which we know 
must have grown on the land, are in some cases below the 
low-tide mark. The shore lines of lakes which once existed 
in the interior, but have now disappeared, also give evidence 
of land movement. Since they were formed on the margin 
of a level body of water, they must have been horizontal ; 
but in some cases these ancient shore lines are no longer 
horizontal. Other evidences might be brought forward in 
proof of a change in the relation between the sea level and 
the land. 

It may be asked whether this is proof of changes in the 
level of the sea, or of land movement. While there is reason 
to believe that there have been changes in the sea level, the 
evidence is conclusive that the greater number of these 
changes in relation of land to sea, are due to actual move- 
ments of the land. Without entering into this subject in 
detail, it may be stated, that the most conclusive evidence 
that this change is due to land movement, is the fact that 
many of the rocks, which we know were formed as nearly 
horizontal layers in the ocean, are now found in mountains 
in a folded and often broken condition. 

Disturbance of the Rocks. — In many cases, the rocks 
that have been raised from the sea, to form a part of the 
continent, are still in nearly horizontal positions (Figs. 90 



208 



PHYSICAL GEOGRAPHY 




Fig. 90. 
Horizontal rocks on the plains of Kansas. 



and 133 and Plate 
28) . They have been 
bodily raised with 
very little disturb- 
ance. In mountains, 
and less prominently 
elsewhere, the rocks 
have been moved 
from their horizontal 
position, and caused 
to assume inclined 
attitudes, which are 
often very complex. These changes commonly assume one 
of two forms, either (1) fold- 
ing or (2) breaking, which 
we call faulting. 

Even the most brittle of 
rocks may be folded. The 
cause for the folding usually 
acts so slowly, and the rocks 
are under such pressure from 
above, that they bend, rather than break, when subjected 

to a strain such as that which 

comes from contraction of the 

interior. A simple kind of fold 

is that known as the monocline 

(Fig. 91), where the rocks are 

inclined in only one direction. 

When they are bent up in the 

form of an arch, the folds are 

si s I gl known as anticlines (Fig. 92), 

FlG 92 and the corresponding down 

Anticline. fold is known as the syncline 




Fig. 91. 
A monocline fold. 




THE CRUST OF THE EARTH. 



209 



(Fig. 93). These may be no 
more than a few inches across 
the base, or they may have a 
width of several miles, with a 
length of perhaps a score of 
miles. 

Among mountains there is 
often an extremely complex sys- 
tem of disturbances, the nature 
of which can best be under- 





Fig. 93. 

Syncline. 

stood by an examination 
of the accompanying fig- 
ures. At times the folds 
are very regular (Fig. 94), 
but usually they are un- 
symmetrical (Fig. 95). 
They are generally ridge- 



Fig.. 94. 

Photograph of an anticline near Hancock, 
W. Va. 

like, and in the direction of the 
ridge they gradually lose in size 
and finally disappear altogether. 
The direction in which these rocks 
enter the earth is known as the 
dip, while a horizontal line at right 
angles to this, is known as the 
strike (Figs. 92 and 93). If we 
considered one side of the gable 
roof of a house to represent an 
inclined layer of rock, the pitch of 




Fig. 95. 

Photograph of a fold in the 
rocks, Quebec, Canada. 



210 



PHYSICAL GEOGRAPHY. 




Fig. 96. 
Photograph of a fault in Arizona 

along a plane which 
result of the fault- 
ing, one side is left 
higher than the other 
(Figs. 96 and 97). 
Sometimes the fault 
plane is nearly ver- 
tical, and sometimes 
nearly horizontal ; 
but it is usually 
inclined at a high 
angle. The amount 
of movement of the 
rocks, varies from a 
fraction of an inch 



the roof would represent 
the dip, and the ridge- 
pole, or any line parallel 
to it, the strike. 

In some cases the 
rocks break or fault, in- 
stead of folding (Fig. 
96), and some folds grad- 
ually change to faults. 
There is much complex- 
ity in faulting, particu- 
larly when the break 
extends across rocks 
that have already been 
folded, and no more can 
be done here than to de- 
scribe the simplest kind 
of fault. The rocks break 
is known as the fault plane ; and as a 




Fig. 97. 
Photograph of fault in glacial clay, Massachusetts. 



THE CRUST OF THE EARTH 211 

to several thousand feet. In the latter case the movement 
did not all take place at once, but was the result of numerous 
slippings, perhaps continued for a long period of time. It 
is probable that in some mountain regions the rocks are even 
now being faulted ; and in some cases the signs of present 
movement can be seen, particularly after earthquake shocks 
(Fig. 247). 

Volcanic Action. — In many parts of the world, particularly 
in some of the higher mountains, molten rock and frag- 
ments of rock are reaching the surface through openings 
that pass down into the earth, probably to a depth of several 
miles. Usually these ejected materials build a cone which 
we know as a volcano (Fig. 234). The molten rock flows 
down the side of the cone as a lava flow and solidifies into 
rock. The fragments are usually porous like ash, and in 
large measure this volcanic ash or pumice also collects near 
the outlet of the volcano. Some volcanoes send forth one of 
these and some the other, while most eject now one and now 
the other. 

Some of the volcanic eruptions are very violent, while 
others are quite gentle, and at times the ash is sent to 
great distances in the air. The lava flows often extend to 
a distance of many miles, deluging the surface over great 
areas. In some cases the lava comes to the surface through 
great cracks, flooding thousands of square miles of country. 
In earlier geological ages volcanoes existed in parts of the 
world where they are now absent, and in such places we 
sometimes find the lava flows at present on the surface. 

Not only are these molten materials sent to the surface, 
but they are found to be intruded in many rocks. Since the 
lava comes from below, it must pass through the strata of 
the crust, and in many cases it solidifies there as injections. 
The tube, through which the lava passes on its way to the 



212 



PHYSICAL GEOGRAPHY. 



crater of the volcano, becomes filled with solid lava when 
the volcanic action ceases ; and sometimes it tries to reach 
the surface along other planes, breaking the rock open and 
filling the cracks with lava, forming dikes (Fig. 98). These 
are very abundant in regions of volcanic action, and they 
often occur in places where such action was once present, 
being the roots of old volcanoes. Such dikes are extremely 

abundant in New England, 
where they may be seen 
in great numbers cutting 
across the rocks of the 
seashore. 

In some of the deep parts 
of the earth, in the center of 
mountains, these intruded 
masses are of great size, 
sometimes miles in diam- 
eter. These great bosses 
of intruded materials are 
illustrated by the granite 
areas ; for these rocks were 
formed in this way, and 
are now exposed at the 
surface because the moun- 
tain cover has been worn 
away. These great masses of molten rock, intruded into 
parts of the earth at depths of a few thousand feet, bring to 
these parts of the crust a greater heat than belongs there, 
and cause many peculiar changes. 

Rocks of the Earth's Crust. — We have no means of 
knowing the condition of the earth at depths greater than 
a few thousand feet ; but the rocks at the very surface are 
quite well known. There are three great groups of such 




Fig. 98. 

Photograph of a dike crossing granite, 
Cape Ann, Mass. 



THE CRUST OF THE EARTH. 213 

rocks, known as igneous, metamorphic, and sedimentary. 
The former come from within the earth, and reach their 
places in the crust as molten rock ; the second kind includes 
those which have been changed or metamorphosed, often 
by heat. This heat has been derived either from intruded 
volcanic rocks, or from friction accompanying the folding of 
mountains. The third group includes those rocks which 
were formed in water, mostly in the ocean. 

Igneous Mocks. — When the igneous rocks come from 
below they are molten, and the elements of which they are 
composed are not definitely united to form minerals. As 
they cool, the elements tend to unite to form definite com- 
pounds, which are minerals. Such rocks are therefore crys- 
talline, for they are composed of crystalline minerals. Since 
the chemical composition of the lavas varies in different 
places, there is much difference in the rock that is formed. 
Some are black, like the trap of the Palisades of the Hudson, 
or like the basaltic lava of the volcanoes of the Sandwich 
Islands, while others are nearly white. The minerals that 
are most common in these rocks, are quartz, feldspar, horn- 
blende, and mica. 1 

If a saturated solution of salt in hot water be allowed to 
cool suddenly, the salt forms one mass of small crystals ; but 
if several hours be allowed to elapse in the cooling, the crys- 
tals are much larger. Just so in these igneous rocks ; and 
as a result of this, some lavas are of very fine grain, and 
even glassy (known as obsidian or natural glass), while 
others are moderately coarse, and still others very coarse. 
Ordinary lava is fine grained because it cools rapidly at the 
surface, while the intruded rocks, such as granite, are much 

1 It does not seem profitable to describe these minerals or the rocks. If 
the students are not already familiar with them, it would be well to have 
them study specimens ; but mere descriptions are of little avail. 



214 



PHYSICAL GEOGRAPHY. 




coarser, because they could not cool so rapidly. Therefore 
igneous rocks vary in two ways, in coarseness and in chem- 
ical composition, and hence in mineral constitution. All of 
these varieties are given names, but their study belongs to 
geology. 

Metamorphic Mocks. — Though they were not molten, 
metamorphic rocks resemble the igneous in the fact that 

they are formed through the 
partial agency of heat, and in 
the fact that they are crystal- 
line. They are the least im- 
portant group, but in some 
places, such as New England 
and Canada, they are the most 
common of rocks. They are 
usually banded or foliated, and 
these bands are often greatly 
contorted (Fig. 99). Some of 
them are known to be the altered forms of other rocks, while 
the original condition of others cannot be told. We know 
that marble is the altered form of limestone, slate is meta- 
morphosed from a clay rock, etc. ; but the two most common 
metamorphic rocks — gneiss and schist — cannot usually be 
traced to their original condition. They are generally very 
hard rocks. 

Sedimentary Mocks. — The most important of the groups 
is that of the sedimentary rocks, which are mostly sedi- 
ments formed in the ocean. They may be divided into three 
classes, — mechanical, chemical, and organic. The organic 
rocks are formed from the remains of animals or plants, the 
coal illustrating the latter and limestone the former. The 
great ocean deposit of Globigerina ooze (page 164), and 
the coral reefs, are organic sediments. Chemical sediments 



Fig. 99. 

Contorted limestone. 



THE CRUST OF THE EARTH. 



215 



are not of sufficient importance to occupy space here, but 
the most important group is the mechanical. 

The rocks of the earth's surface are being destroyed by 
various means, and the fragments are being transported 
toward the sea. Since some of the minerals cannot with- 
stand the action of the weather, the rocks actually decay 
and form fragments ; and as they change and crumble, the 
rock falls to pieces, thus making the beginning of a soil. 
Every rain takes some of these pulverized rock particles and 
carries them to a stream, where they begin their journey 
to the sea (Fig. 122). To these are added others which 
the stream takes 
from its bed; and 
in the ocean there 
are added those 
that the waves 
rasp from the 
land. In the 
ocean these ac- 
cumulate in lay- 
ers, the coarsest 
where the waters 
are in most rapid 
motion, and the 
finest where they 
are so still that 
the particles may settle. The coarser rocks with pebbles, 
such as those of the beaches, are known as conglomerates ; 
the very finest produce clay rocks, such as the shales ; and 
the intermediate sandstones are composed of sandy grains 
of the very durable mineral quartz. 

Deposition of Sedimentary Rocks. — Reaching the ocean, 
these rock fragments are strewn over the bottom of the sea, 




Fig. 100. 
Stratified shale rocks in a gorge near Ithaca, N.Y. 



216 



PHYSICAL GEOGRAPHY. 



particularly near the coast, because here the ocean waters are 
so quiet that the particles must settle. In quiet bays, very 
fine-grained rocks may be deposited close to the shore ; but 
on more exposed coasts, the sediments of the shore line are 
coarse-grained, and as the distance from 
the coast increases, they become finer 
in texture. Since the ocean bottom is 
usually nearly level, these fragments are 
spread out in layers which are nearly 
horizontal, though where the bottom is 
inclined, the layers are inclined with it. 
Sometimes the supply of sediment varies, 
either in amount or in kind, and so one 
layer may be deposited on another ; and 
this gives the stratification that is so 
characteristic of most sedimentary rocks 
(Fig. 100). We may have a layer or 
stratum of sand resting on one of clay, 
and upon this a layer of limestone, etc. 
(Fig. 101). 

Sedimentary rocks are now being 
formed over the entire floor of the ocean ; 
but at a greater distance than a few score 
of miles from the land, the sediments for 
the most part are organic. The greater 
part of the rocks of the land are sedi- 
mentary in origin ; and most of them furnish evidence that 
they were formed in the ocean near the shore. This proves 
that they must have been elevated from the sea ; and we 
know full well that the continents are largely built of 
materials that were formed in the ocean not far from the 
shore. Sometimes these rocks have a thickness of thou- 
sands of feet, and yet they are made up of sediments that 




K*^ 



Shale 

Shale 

Sandstone 

Shale 

Conglomerate 

Shale 

Sandstone 

Shale 
Conglomerate 

Shale 

Shaly 
Sandstone 



Shale 
Sandstone 

Limestone 

Fig. 101. 

Section showing alter 
nation of strata. 



THE CRUST OF THE EARTH. 



217 




Fig. 102. 
An unconformity in horizontal rocks. 



were laid down in the shallow waters near the coast. The 
only way in which this could happen is by a continued 
sinking of the bottom. Therefore the sedimentary rocks 
teach us that parts of the sea bottom continued to sink for a 
long time, and were then elevated to form continents. 

Other movements of the 
crust are also shown by 
some of these rocks. At 
times there are unconform- 
ities (Figs. 102 and 103) : 
that is, rocks made in the 
sea, rest on other sea- 
formed strata which were deposited at an earlier period, 
and have since been land. Thus we have in these cases, 
(1) deposit in the ocean, (2) elevation to land, (3) depression 
beneath the sea, and (4) a second elevation. In some cases 
there are numerous such unconformities, showing successive 
changes. These, and other facts, prove that the crust of the 

earth is almost con- 
stantly in movement. 
Consolidation of 
Sedimentary Rocks. 
— The rocks of the 
sea are soft and un- 
consolidated, while 
those of the land are 
generally hard and 
compact. The consolidation of rocks is a simple process, 
generally resulting from heat, pressure, the deposition of 
some cement, or a combination of several such causes. In a 
hydraulic press we can consolidate clay ; and in a similar 
way, the great weight of the strata of the crust, furnishes 
the necessary pressure for the natural consolidation of rocks. 




Fig. 103. 

An unconformity in inclined rocks, 
land surface. 



A, B, old 



218 PHYSICAL GEOGRAPHY. 

Bricks are consolidated by heat, and in the earth heat often 
acts in a similar manner. All rocks in the earth are filled 
with water which is slowly percolating through them. This 
water is dissolving substances from one place and depositing 
them in others, and in this way many rocks are being con- 
solidated. Carbonate of lime and some compound of iron, 
are the common rock cements ; and these, perhaps aided by 
one of the other causes, bind the rock particles together. 

Geological Chronology. — By a study of the rocks, the main 
facts of geological history have been determined in a more 
or less satisfactory manner. We know something of the 
history of the globe, and the rocks form the pages and chap- 
ters of this history. The rock record is often very imper- 
fect. Some pages, and at times entire chapters, are missing ; 
but enough still remains to furnish a basis of value. One 
thing shown, is that the world is very old, and that no 
statement of the history in years or centuries is possible. 
Therefore there is no chronology of the kind that we are 
accustomed to use in recording human events. 

In many of the sedimentary rocks there are fossils, which 
are the entombed remnants of animals and plants that lived 
when these rocks were formed (Fig. 104). If 1000 feet 
of rocks are found, one laid down upon the other, and if 
these contain fossils, there is preserved a record of some of 
the organisms that lived while these rocks were being depos- 
ited. By a very careful study of the fossils of various parts 
of the earth, a nearly continuous record of the life of the 
globe has been obtained, from near the beginning of life to 
the present time. It is found that in the lowest rocks — that 
is, in the oldest — the animal remains are only of low types. 
At first there were no land animals and plants ; and in the 
sea, the only animals were of types lower than the true 
fishes. The fishes appeared, then reptiles, birds, and mam- 



THE CRUST OF THE EARTH. 



219 



mals in succession ; and this evolution from lower to higher 
forms is noticed even among the subdivisions of life. 

Therefore, upon examining the fossils from a rock, a geol- 
ogist can tell in what part of the earth's history they lived, 
and to what stage in this history the rock belongs. It is 
like the study of prehistoric man, which is based on the 
implements he used. Certain kinds of stone implements 








Fig. 104. 
Photograph of a rock containing fossils. 

mark the palaeolithic age, others the neolithic ; bronze 
implements mark a higher stage, etc. This does not mean 
an age in any sense in which years are used, but rather 
a stage. One of these stages may represent a thousand 
years, another several thousand ; but each one represents a 
stage different from that which preceded and succeeded. 

So it is with the geological chronology. We have abso- 
lutely no basis for division into periods of years ; but we can 



220 



PHYSICAL GEOGRAPHY. 



divide the history into stages, each stage representing some 
advance in the development of life on the globe. For this 
purpose, names are used to signify the stages, as is indicated 
in the table below, which is a simple one from which the 



TABLE OF GEOLOGICAL AGES. 



CENOZOIC 
TIME. 

Age of 
mammals. 


Quaternary. 


Man assumes importance, particularly in 
the upper part. In the first half the Gla- 
cial Period prevailed. 


Tertiary. 


Mammals develop in remarkable variety, 
and to great size, while reptiles dimmish. 


MESOZOIC 
TIME. 

Age of reptiles. 


Cretaceous. 


Birds begin to become important, reptiles 
continue, and higher mammals begin. 
Land plants and insects of high types. 


Jurassic. 


Beptiles and amphibia continue to be 
predominant. 


Triassic. 


Amphibia and reptiles develop remark- 
ably. Mammals of low forms appear. 


PALAEOZOIC 
TIME. 

The age of 
invertebrates. 


Carboniferous. 


Land plants assume great importance. 


Devonian. 


Fishes begin to be abundant. 


Silurian. 


Invertebrates prevail. 1 


Cambrian. 


No forms higher than invertebrates. 


In part 

AZOIC TIME. 

No fossils known 


Archean. 


Mostly metamorphic rocks, perhaps in 
part the original crust of the earth. 



1 Invertebrates of course continue down to the very present ; but until the 
Devonian, they were the most important group. The same is true of fishes, 
which begin to be abundant in the Devonian, but continue down to the present. 



THE CRUST OF THE EARTH. 221 

subdivisions are omitted. Each of these ages represents the 
lapse of immense periods of time, perhaps hundreds of 
thousands of years ; but no interpretation of years is to be 
placed upon them, nor should it be assumed that they are of 
equal length. The Carboniferous represents the stage in 
the earth's history when plants had reached a certain type 
of development upon the land, etc. 

Age of the Earth. — As has been said, we have no basis for 
an estimate of the age of the earth. By some scientists esti- 
mates have been made upon one basis or another, and these 
have ranged between 3,000,000 and 2,400,000,000 years, 
though the majority have estimated a few hundred million 
years. Since these estimates were made by very different 
men, upon entirely different facts, they have the one great 
value that they prove the great age of the earth. 

One cannot go far in the study of geology without being 
convinced by the overwhelming evidence that the earth is 
exceedingly old. To attempt to explain the phenomena of 
the earth's surface upon the basis of single years or centu- 
ries, would be as fruitless as would be the attempt of the 
astronomer to explain the facts of the solar system on the 
supposition that the planets were at distances of a few 
thousand miles. The only way to have the force of this 
statement impressed in all of its fulness, is to study the earth 
with the eyes of a geologist ; and in a study of this nature 
only the beginning of this can be attempted. Still it is 
necessary that this fact should be accepted at the outset. 
Just as the student of astronomy gazes at the stars, and, 
upon faith alone, accepts the statement that- these bodies lie 
millions and even billions of miles from him, so the student 
of geology or physical geography must commence the study 
of the earth with the belief that the history which it has 
passed through has occupied not years, nor thousands, nor 



222 PHYSICAL GEOGRAPHY. 

even hundreds of thousands, but millions and probably hun- 
dreds of millions of years. The evidence is overwhelming, 
and no geologist finds reason to doubt it. 

The gorge of Niagara, 200 or 300 feet deep, and 7 miles 
long, has taken not far from 10,000 years for its formation ; 
how much longer was the time occupied in forming the canon 
of the Colorado, whose length is 300 miles, and whose depth 
in places is over a mile ! Yet these were formed in late 
stages in the development of the continent. 

We watch a volcano for a century, and, at the end of that 
time, find its general form to be the same as at the begin- 
ning ; yet most of the volcanic cones of the world were 
begun not earlier than the commencement of the Tertiary. 
Studying the rate of deposit of the sedimentary rocks of the 
ocean, we find that, even when the deposit is rapidly made, 
but a few feet are laid down in a single century ; yet, in 
some places, many thousand feet of rocks have thus been 
deposited, one layer upon another. In the Appalachian 
Mountains there are fully 40,000 feet of these strata, and 
they were all formed in the Palaeozoic. How many scores 
of centuries do these represent ! 

This, and other evidence equally striking, is what has 
driven the geologists to the conclusion (for a long time 
opposed, as was the present astronomy when first proposed) 
that the age of the earth is incalculable, but great, — a conclu- 
sion now quite universally accepted. It is the basal concep- 
tion of geology, and must be accepted at the beginning. To 
it must be added the conception of the fact that the earth is 
changing. These changes, so slow as to be almost impercep- 
tible in a single lifetime, when allowed long periods of time 
for their action, will produce the most profound and stupen- 
dous revolutions. From this time on we will study the crust 
of the earth as a thing of constant change, and of great, but 



THE CBUST OF THE EARTH. 223 

indefinite age. The present is but one stage in its history : 
there has been a past, and there will be a future, just as is 
the case with the history of man himself. 



REFERENCE BOOKS. 

LARGER BOOKS OF REFERENCE. 

Geikie. — Text Book of Geology. Macmillan & Co., New York. Third 

edition (revised), 1893. 8vo. $7.50. (The most complete English text 

book.) 
Dana. — Manual of Geology. American Book Co., New York. Fourth 

edition (revised), 1895. 8vo. $5.00. (The standard American reference 

book ; thoroughly revised to date.) 
Le Conte. — Elements of Geology. American Book Co., New York. Revised 

edition, 1891. 8vo. $4.00. (A very valuable book of reference. ) 

SMALLER TEXT BOOKS. 

Geikie. — Class Book of Geology. Macmillan & Co., New York. Third 
edition, 1892. 12mo. $1.10. 

Jukes-Browne. — Handbook of Physical Geology. Macmillan & Co., 
New York. Second edition, 1892. 12mo. $1.75. 

Le Conte. — Compend of Geology. American Book Co., New York, 1894. 

12mo. $1.20. 
Dana. — Text Book of Geology. American Book Co., New York. Fourth 

edition, 1884. 8vo. $2.00. 
Winchell. — Geological Studies. Griggs, Chicago. Fourth edition, 1892. 

12mo. $2.50. 

This list contains only a few of the many excellent text books of geology ; 
and others are referred to at the end of the next chapter. 

In some of the states of the Union, there are geological surveys which have 
published reports in which one may often find a description of his own 
region. Among others, the following states have recently had such surveys : 
New York, New Jersey, Pennsylvania, North Carolina, Georgia, Alabama, 
Mississippi, Texas, Arkansas, Ohio, Michigan, Minnesota, Missouri, Kansas, 
Iowa, South Dakota, and California. Where the reports cannot be obtained 
from the state geologist, they can often be found in second-hand stores. 



CHAPTER XIII. 

DENUDATION OF THE LAND. 

Underground Water. — When rocks are deposited in the 
ocean, the crevices between the particles of sediment are 
filled with water. In even the densest of rocks there are 
cavities, and through all of these, water is slowly percolating 
as underground water. Added to the supply originally in 
the rocks, there is a constant body of water entering at the 
surface. When rain falls upon the land, a part is returned 
to the air by evaporation, a second portion flows away as 
surface water, and a third part sinks into the ground. This 
last portion commences an underground journey through the 
strata, in the course of which much work is done. It moves 
along the larger crevices, and also slowly passes through the 
very rock itself. That this water is actually present in the 
strata, is shown by the fact that wells may be constructed in 
them ; and even in the deepest mines, water is found to be 
present in the rocks. 

Some minerals are soluble in water, and the hardness of 
certain waters is due to the fact that they contain mineral 
matter in solution. All underground water is engaged in 
this work of dissolving rock materials. While pure water 
has but little power of solution for ordinary minerals, when 
it is supplied with certain impurities its solvent power is 
greatly increased. There are many substances which add 
power to this percolating water, but those which are most 
commonly present, are the various acids supplied by decaying 

224 



DENUDATION OF THE LAND. 



225 




ik„ - /til "• 1 '.' I '! • ' . * 


j 

'* j 

j 

:-.:■ . -1 

j 

1 


! 1 . , . . '-"- 


1 

! 

... .';4j 



vegetation. The humous and humic acids and carbonic acid 
gas are most commonly present in underground water ; and 
armed with these, it possesses great solvent power. When 
the water has percolated to a considerable depth in the earth, 
its temperature is so raised that its power is greatly increased. 
In some cases it obtains a temperature higher than the boil- 
ing point at the surface ; and then it becomes a powerful 
solvent, partic- 
ularly if it is 
armed with 
acids or alkalies. 
When it reaches 
the surface in 
the form of a 
spring, we very 
often find proof 
that under- 
ground water is 
engaged in this 
work of solu- 
tion. Many of 
these are min- 
eral springs, and 
at times, deposits 
of iron, or other 
substances, are 
made where the water reaches the surface. When hot water 
escapes at the surface, as is the case in the geyser region 
of the Yellowstone Park, extensive chemical deposits of rock 
are sometimes formed around the springs (Fig. 105). The 
reason for the deposit of these substances, is sometimes that 
the temperature of the water is lowered, and its solvent 
power thereby decreased; in other cases it is due to the 




Fig. 105. 

Deposits of carbonate of lime, Pulpit terrace, Mammoth 

Hot Springs of Yellowstone Park. 



226 PHYSICAL GEOGRAPHY. 

escape of certain gases which gave to it much of its power ; 
and it is often the result of chemical changes in the presence 
of the air. Even in the earth, for one reason or another, the 
water at times deposits some of its dissolved load. This is 
one of the ways in which rocks are cemented ; and it appears 
to be one of the causes for the formation of some of the 
valuable mineral deposits. 

Underground water is also engaged in the work of chang- 
ing some of the minerals of the rocks. It actually causes 
a decay of some minerals, and brings about very important 
changes in others. This is one of the ways in which the 
rocks are broken into fragments, and soils formed. This 




Fig. 106. 
Diagram to illustrate the formation of caverns. 

work of underground water is not confined to the surface 
layers, but extends to considerable depths in the earth. 
However, from our present standpoint, the most important 
changes are those which are produced nearly at the surface, 
and these are again referred to below. 

The Formation of Caverns. — Limestone is one of the most 
soluble of the rocks ; and in many of the regions where this 
exists, the solvent action of underground water goes so far 
as to actually dissolve cavities in the strata (Fig. 106). It 
sinks into the ground through depressions, or sink holes 
(Fig. 107), and passes along planes of weakness, which it 
enlarges by solution ; and in some cases, this underground 
water assumes the form of true subterranean rivers, which 



DENUDATION OF THE LAND. 



227 



are sometimes 
several miles in 
length. The cav- 
erns (Fig. 106) 
thus formed, are 
very irregular ; 
and some, such 
as the Mammoth 
Cave of Ken- 
tucky, and Lu- 
ray Cave, have 
been explored 
and opened to 
tourists : but there 




Fig. 107. A sink hole in a limestone region. 



are thousands 




Fig. 108. Stalactites in cavern of Luray. 



which have never been 
entered. 

In some of these 
caves, the water that 
percolates through the 
roof, deposits columns 
and pendants of car- 
bonate of lime, which 
often produce most 
beautiful effects. 
When these reach 
from the roof they are 
known as stalactites 
(Fig. 108); and when 
they extend from the 
floor, they are called 
stalagmites; while by 
the junction of these, 
columns are often 
formed from floor to 



228 



PHYSICAL GEOGRAPHY. 



roof. They are formed because on entering the cave the water 
loses some of the carbonic acid gas which gave to it its sol- 

vent powers, and thereby has its 

ability to hold in solution de- 
creased. By the gradual lower- 
ing of the land, the roofs of 
these caverns are sometimes de- 
stroyed, and the streams that 
occupy them are changed to sur- 
face rivers. Where a part of the 
roof remains, a natural bridge is 
sometimes formed (Figs. 106 and 
109). 

Springs and Artesian Wells. — 
Underground water' often finds 
channels of escape to the surface ; 
and where it reaches the surface, 
springs are produced. This escape 
may be along fault planes, or other 
breaks in the rocks (Fig. 110), or it may be at the outlet 
of a subterranean stream which passes through a cavern 
(Fig. 131); but the majority of springs occur where a 




Fig. 109. 
The Natural Bridge, Virginia. 




Fig. 110. 
A spring formed along a fault plane (/, 5) ; (a, a) impervious layers; (6) porous 
stratum. Arrows show the course followed hy the underground water leading 
to the spring (s) . Water passes up the fault plane from / to s. 



DENUDATION OF THE LAND. 



229 



loose-textured rock rests upon a less permeable one, and 
where this junction is exposed at the surface (Fig. 111). 




Fig. 111. 
Hillside spring (s) at junction of permeable layer (a) and impervious layer (b). 

This is particularly liable to happen on hillsides where a 
layer of sand rests upon a stratum of clay. 

In the earth, certain strata are more permeable to water 
than are others; and under some 
circumstances the conditions fa- 
voring the production of arte- 
sian wells (Fig. 112) may be 
present. Sandstones are the" 
most permeable of rocks, and 
when a sandy layer crops out at 
the surface, the water readily 
soaks into it. If such a layer is 
covered and underlaid by a more 
dense rock, such as a clay stra- 
tum, the water that enters the 
sandy layer is in large measure 
imprisoned within it. If under 
such conditions the strata dip 
into the earth, the water in the 
sandstone passes down this layer 
between the two enclosing walls. 
As a result of the weight of the 
column of water in the stratum, it is under a considerable 




Fig. 112. 
Artesian well. 



230 



PHYSICAL GEOOBAPHY. 



pressure ; and this is sufficient to force it upward toward 
the surface, to a height nearly as great as that of the place 
where the water enters the ground. 

If this water-bearing layer is pierced by a well-boring, 
the water will rise in the well as high as the pressure 
can force it ; and if the place at which the well is bored, 




Fig. 113. 

Conditions favoring artesian wells (c, c, c), where the rocks are inclined in a 
single direction. Porous sandy layer (a), impervious strata (b, b). 

has a lower elevation than the water head, the water from 
the stratum may reach the surface as a fountain, forming 
an artesian well (Fig. 113). When this condition is en- 
countered in a syncline, there are two water heads, and this 
greatly favors the formation of an artesian well (Fig. 114). 




Fig. 114. 

Artesian wells (c, c, c) , where rocks are folded into the form of a syncline ; 
(a, a) porous layer between two impervious layers (b, b). 



In eastern Texas, there is a water-bearing stratum extending 
over a great area (Fig. 113), which has been tapped at nu- 
merous places, and which furnishes abundant water supply 
for several cities ; and the same is true of South Dakota 
and elsewhere. In many parts of the west, artesian wells 
are very useful for purposes of irrigation. It often happens 



DENUDATION OF THE LAND. 



231 



that the water does not rise quite to the surface, and then 
pumps are necessary, the pumping often being done by wind- 
mills. 

Durability of Rocks. — There is a great difference in the 
ability of rocks to withstand the action of the agents which 
are tending to destroy them. Some, such as granites, are 
very hard ; others, such as limestones and shales, are soft. 
Many rocks that are hard 
are chemically weak, and 
their minerals are easily 
dissolved, or are readily 
altered. By these proc- 
esses, such strata are 
caused to decay and crum- 
ble. Some rocks are loose 
in texture and readily en- 
tered by percolating water, 
while others are dense and 
quite impermeable. Other 
things being equal, the 
latter are less easily de- 
stroyed than those that are 
loose in texture. Some 
which are mechanically 
hard are readily destroyed 
by chemical means. In 
the later pages, when a 

hard rock is mentioned, the term is used not merely in the 
mechanical sense, but as a synonym of resistant. 1 All rocks, 
no matter how resistant they may be, are capable of being 

1 That is to say, a hard or resistant rock is one which withstands all 
attacks, whether mechanical or chemical, more successfully than less durable 
rocks, as explained in the next section. 




Fig. 115. 

Kock pillars, Garden of Gods, Colorado. 
Soft rock capped by a harder one and 
hence protected from destruction. 



232 



PHYSICAL GEOGRAPHY. 




Plate 20. 

Earth columns, New Mexico. Illustrating the greater resistance of the thin, hard 

layers in soft clay. The beginning of the formation of rock pillars. 



DENUDATION OF THE LAND. 



233 



destroyed ; but there is a difference in their power of resist- 
ing destruction (Fig. 115 and Plate 20). 

Weathering. — When exposed to the air, or to the weather, 
rocks are destroyed by various agents which may be included 
under the general heading of weathering. These agents are 
both chemical and mechanical. Already some of the chemical 
changes have been noted in the section on underground water. 
Soluble minerals are taken from the rocks, and those that are 
left are then less firmly bound together. The same result is 




Fig. 116. 
The crumbling of granite by disintegration of the minerals. 



brought about by the change of minerals during the passage 
of water through them. Usually the change leaves the rock 
less firm than it was at first, and it often produces a clayey 
product in the place of the firm mineral that was originally 
present. These chemical changes are particularly liable to 
happen in the crystalline rocks, which were formed by the 
aid of heat (Fig. 116). When exposed to the air and water, 
the minerals that cooled from a molten condition are found 
to be unstable and liable to change. Some minerals, such as 
quartz, resist this destruction, and this is why we have fresh 



234 PHYSICAL GEOGRAPHY. 

quartz grains in sandstones that have been produced by the 
decay of rocks in which quartz was one constituent. The 
clay of such rocks as shale is mostly the product of this 
rock decay. Another result of these changes is to furnish 
dissolved mineral substances to river water, and hence to 
the sea. 

Of the mechanical agents, perhaps the most important is 
that of change in temperature, which, however, affects only 
the very surface rocks. In the regions which experience 
great temperature ranges, the rocks become warmed during 
the day and cooled at night. This introduces an alternate 
expansion and contraction, which causes fragments to be 
split from the rock surface. If the temperature descends 
below the freezing point, as is the case in the high temperate 
and arctic latitudes, the water in the rock crevices is frozen, 
and, by the consequent expansion, fragments are pried off. 
This is a very important action on mountain tops (Fig. 224) 
and on exposed ledges in cold countries. A snow cover- 
ing tends to check this action. Naturally, those rocks with 
porous texture are more open to the attacks of frost than 
those which are compact ; and open-textured rocks are also 
more liable to be readily destroyed by percolating water 
than are those of fine and compact grain. 

Plants are also important agents of weathering, and their 
action is both chemical and mechanical. They act chemi- 
cally by furnishing to percolating water many of the sub- 
stances with which it is able to dissolve and alter the 
minerals ; and they also extract mineral matter from the 
soil in water absorbed through the roots. The mechani- 
cal action of plants is mainly that of their roots. These 
enter the rock crevices, and upon growing, enlarge these 
cavities, causing the rocks to crumble (Fig. 117). This 
action may often be seen upon a ledge on which lichens are 



DENUDATION OF THE LAND. 



235 



growing ; and the roots of trees are doing a very important 
work of this nature, because they extend through the soil 
to the rock beneath. 

Even animals are aiding in this work, particularly those 
that burrow in the earth. Earthworms are of great impor- 
tance in this respect, for they are engaged in the constant 
work of pulverizing the soil. The action of the agents 
above described, is not confined to the solid rock, but it is 




Fig. 117. 
Roots of a tree breaking a rock into fragments. 

constantly in progress in the soil, the tendency always 
being to make this finer in texture. 

The results of this action of weathering are most wide- 
spread. All over the land, in nearly every place, the rocks 
are being destroyed by these agents ; and weathering is the 
most important single cause for the destruction of the strata 
and the melting down of the surface of the land. Weather- 
ing is more rapid in some places than in others. On the 
cold mountain tops, its action is rapid (Fig. 224), as it is also 
in regions of moisture. On the other hand, in arid regions 
where rain is uncommon, weathering is relatively slow, as 



236 PHYSICAL GEOGRAPHY. 

it is also in regions where a deep soil covering protects the 
rocks. Upon exposed ledges, weathering is rapid ; and this 
is particularly true of cliffs, where the fragments drop to 
the base in the form of a talus (Figs. 118, 122), leaving the 
rock-face bare to future attacks. Then also, weathering is 
more rapid in some kinds of rocks than in others. 




Fig. 118. 
Talus, valley of Rio Grande, New Mexico. 

The great result of weathering is the lowering of the land 
surface ; and in the course of the vast ages of geological 
time, not only hills, but mountains and volcanoes, have been 
destroyed mainly by the action of this slow melting away of 
the rocks. By the folding and elevation of the strata, new 
tasks are constantly set before these agents, and we may 



DENUDATION OF THE LAND. 



237 



say that there are two opposing forces at work, one tending 
to increase land elevations, the other to lower them. In this 
combat, elevation has excelled ; and as a result we have a very 
irregular land surface. If there had been no weathering, 
the land elevation would have been vastly greater, but the 
surface of the land would have been much more regular. 
If there had been but one elevation, and that at the begin- 
ing, the land would have been worn down to a nearly 
level plain. 

Had weather- r 
ing been the 
only agent of 1 
destruction, the 
result would | 
have been very 1 
different. With | 
nothing to re- | 
move the frag- § 
ments, the solid 1 
rock would 1 
have been cov- 
ered with a soil 
that would have 
protected the 

strata from further destruction ; and the longer it acted, the 
less its power would be, the process being, as it were, self- 
destructive. There have been other agents at work, and 
these have served to remove the disintegrated rock frag- 
ments. Some of these agents, being chemical, have carried 
the material away in solution, others have acted mechani- 
cally. These are described under the following heading 
of erosion. 

Among the results of weathering, one of prime importance 




Fig. 119. 
Disintegrated rock, forming residual soil. 



238 



PHYSICAL GEOGBAPHT. 





to man is the formation of soil. In many parts of the earth 
the soil is the result of rock disintegration (Fig. 119); and 
in some places, particularly in the tropics, this residual soil 
(so called because it is largely composed of the insoluble 
residue of rock decay) has a depth of 100 or 200 feet. 
In this country it is of particular importance in the Southern 
States, the soil of the Northern States being largely the result 
of glacial action, and being a transported soil. Another 

important effect of 

this rock decay, is 
that it furnishes to 
rivers the larger 
part of the sediment 
load with which they 
are able to cut their 
channels, the rock 
particles being used 
as cutting tools. 

Agents of Erosion. 
— In certain places, 
various agents are 
at work cutting into 
the rocks and re- 
moving materials, either chemically, mechanically, or both. 
The most important of these are wind, rain, percolating 
water, rivers, oceans, and glaciers. 

Wind HJrosion. — In some places the action of the wind 
is of considerable importance ; but in most regions a forest 
or grass covering protects the rock and soil from its action. 
On the seashore the blowing of the wind drives sand about, 
and with it often batters the rocks in a manner analogous to 
the sand blast with which glass is ground. On some of the 
sandy islands of the seacoast, the window panes are some- 




Fig. 120. 
Sand dunes, Cape Ann, Mass. 



DENUDATION OF THE LAND. 239 

times transformed to ground glass. Many narrow islands 
along the seashore are built above sea level by the action 
of the wind upon the sand, which is washed into the form of 
bars by the waves ; and on some coasts this sand is driven 
inland, where it accumulates as hills, known as sand dunes 
(Fig. 120). In the arid regions, where the soil is not 
covered with dense vegetation (Fig. 121), the winds are 
constantly engaged in the removal of the finer rock frag- 
ments ; and in these places the wind becomes one of the 
most important agents of erosion. Oftentimes the air is filled 
with blown sand, so that even neighboring hills are obscured. 
This natural sand blast 
beats against the rocks, 




and wears them away, re- L^^ss 

moving all the finer par- j 

tides as fast as they fall . . • * * * - ^ 

from the rocks (Figs. 69 *f' ' ' """ * nJ ' 

and 121). < / 1 ^^^^^\C^-'^9 t 

Rain Erosion. — During j 
a rain, the drops that reach Fig. 121. 

the Soil do a slight amount Mo 1 ui Pueblo, New Mexico, a rocky point 
„ . , exposed to wind action. 

of erosion and transporta- 
tion, particularly if they fall upon a hillside. Even before 
the rain gathers into little rills, it does some work of this 
kind ; and when it has formed tiny streams, it commences to 
wash the soil down toward the rivers. This is one of the 
ways in which rivers are supplied with their load of sediment. 
During a rain, one may see this process upon a plowed field 
or on a road. In the forest, and upon turf-covered land, this 
action of the rain is of little importance ; but in dry regions, 
where the soil is not protected, every rain causes the soil to 
creep down the hillsides ; and in the mountains of the arid 
regions, great gravel-slopes are by this means accumulated at 



240 



PHYSICAL GEOGRAPHY. 



the mountain bases. This form of erosion merges into that 
of rivers. In some places (Plates 20, 21, and 29) rain erosion 
has carved the soft clay of the arid lands into a series of fan- 
tastic and remarkable forms. 

Gravity is an important factor in this and other kinds of 
erosion ; but even when unaided by any of the agents of 




Fig. 122. 
River receiving the load from a talus at the hase of a canon wall. 

erosion, gravity alone is in some places an agent of destruc- 
tion. The fragments loosened from cliffs by frost, or other 
agents of weathering, fall to their base and accumulate there 
as talus slopes (Figs. 118, 122, and 219). This is an impor- 
tant source of sediment for rivers, and among mountains, the 
talus slopes are important elements in the topography. 

Percolating Water. — A second part of the rain enters the 



DENUDATION OF THE LAND. 241 

ground ; and aside from the work of rock destruction de- 
scribed above, it does an important work of rock removal. 
This is largely chemical, but partly mechanical. It removes 
soluble substances; and when it again reaches the surface, 
some, if not all of this, is furnished to streams for transporta- 
tion, and thus much of it finds its way to the sea. 

The most important mechanical work, is that of aiding 
the sliding of the soil down the hill slopes. The percolating 
water makes the soil particles slippery, and in some cases 
great masses fall down, forming avalanches or landslides. 
These very frequently occur where a porous layer rests upon 
an impervious one, as for instance when a sand stratum rests 
upon a layer of clay. The clay is lubricated and a slipping 
plane produced ; and then under favorable circumstances, a 
mass of earth falls down. A strong wind blowing through 
the trees may start the slide, or the action of frost, or of a 
heavy rain, may introduce the conditions which are necessary 
for the beginning of the landslide. 

River Erosion. — The subject of rivers is taken up in the 
next chapter, and only a few words need to be given to it 
here. The river is engaged in three great tasks, (1) the 
removal of water from the land, (2) the transportation of 
sediment given to it, and (3) the cutting of its channel. 
Two kinds of material are furnished to it, (1) mineral matter 
in solution, largely supplied by the underground water which 
is tributary to the stream, and (2) fragments of rock furnished 
by weathering. Under different circumstances, the amounts 
of these substances vary greatly. Some streams are clear 
and free from sediment, others are always filled with mud ; 
but most streams are usually clear, and become clouded with 
sediment only after a heavy rain. In some cases, the ma- 
terial carried is in the form of fine mud; in others it is 
pebbles and even large boulders (Fig. 124). All streams 



242 



PBYSICAL GEOGRAPHY. 



carry substances in solution, but some have a little, while 
others carry great quantities ; and in desert regions, the 
rivers are sometimes so full of dissolved substances, that 
the water tastes bitter or salt. 

Armed with its load of sediment, the river cuts the rocks 
of its channel, and deepens its valley ; and by swinging from 

one side to the 
other, it broadens 
the valley slight- 
ly. Thus by river 
erosion, there is 
produced a rela- 
tively deep and 
narrow channel, a 
gorge, or a caiion. 
In arid regions, 
where weathering 
is of little impor- 
tance, this is the 
prevailing type of 
river valley ; but 
most of the val- 
leys of moist coun- 
tries are U-shaped 
rather than V- 
shaped. This is because the action of weathering has caused 
the valley sides to melt back. River erosion deepens, weath- 
ering broadens the valleys (Fig. 123) ; and since the latter 
acts more slowly than the former, when streams begin their 
work, they produce deep, narrow valleys, even in moist 
countries. They cut down much more rapidly than weath- 
ering can broaden, and hence young valleys are gorges ; and 
this is true wherever erosion greatly exceeds weathering. 




Fig. 123. 

Yellowstone Valley, showing the broadening of a 
V-shaped valley by weathering. 



DENUDATION OF THE LAND. 



M3 



The rate of erosion varies with the slope and the volume 
of water in the stream. Where the slope is great, if other 
conditions are favorable, the erosion is rapid ; and where the 
amount of water is great, the erosion is more rapid than 
under similar circumstances with smaller volume. There- 
fore in the same stream, the amount of erosion done during 
its swollen condition, greatly exceeds that done when the 
amount of water is not great (Fig. 124). 




Fig. 124. 

Westfield River, Massachusetts, showing houlders which may be moved 
when the river is swollen. 



The rate also varies with the amount of sediment; for if 
there is no sediment, there are no tools with which to work, 
and clear water can do little work except that of solution, 
which is relatively unimportant. On the other hand, if 
the river is given more sediment than it can dispose of, it 
cannot cut its channel, but must deposit some of its load in 
the valley, as is being done in the lower Mississippi. The 
most favorable condition is that of a moderate amount of 



244 



PHYSICAL GEOGRAPHY. 



sediment. With the hardness of the rocks there is also a 
variation ; for a river cannot cut its channel so rapidly in 
a hard granite as it can in a soft clay. 

From this it will be seen, that the rate and kind of work 
that a stream is doing, varies greatly according to circum- 
stances ; and it follows that river valleys must present very 
different characteristics. Some are narrow, others broad ; 
some deep, others shallow ; some have rapid slope, others 
have a gentle flow, etc. In carving the land, river erosion 




Fig. 125. 

An oceanic volcanic island, showing a cliff produced by wave action in eating 

back into the land. 

is an important agent ; but its importance does not depend 
so much upon the work of cutting it does, as upon the fact 
that it is the agency by which rock fragments, prepared by 
other means, are removed from the land. River erosion and 
weathering are intimately combined in the destruction of 
the land, and in the sculpturing of its surface. 

Ocean Erosion. — The action of the ocean in eroding, is 
confined to the limited area of the immediate coast line ; but 
here it is often very important. The waves are constantly 



DENUDATION OF THE LAND. 



245 



beating on the shore, and battering at the rocks, often with 
terrific force. Armed with sand and pebbles, and even by 
its direct action, the wave is able to wear back even the 
hardest rocks; and in the ocean, islands that were once of 
great size are now only remnants (Figs. 125 and 195). 

On the beaches and on the headlands, rocks are being 
ground into finer particles. The materials thus obtained, 
added to those received from other sources, are removed, 
mainly by the movements of the wind and tidal currents, 




Fig. 126. 
A granite hill rounded by glacial action. 



and distributed over the bottom of the sea near the land. In 
these ways coasts are changed in form, and are ever changing ; 
though here, as in most other geological changes, the work 
is slowly accomplished. On the British coast, where the 
changes have been studied for centuries, it is found that the 
coast line has been very decidedly altered by ocean erosion. 

Grlacial Erosion. — Glaciers are now relatively scarce in 
this country, but at one time they were present in northern 
United States (Chapter XVII.), and they then did consider- 
able work of erosion. Because it is a rigid body, ice acts dif- 



246 PHYSICAL GEOGRAPHY. 

ferently from water. There is no chemical work done, and 
the mechanical work is different ; for the ice exerts a great 
pressure, and, armed with rock fragments, it scours its bed 
in a manner analogous to a great sandpaper. It rounds off 
the surface (Fig. 126) and acts all over its bed, so that if it 
spreads over a country, it scours hills as well as valleys. 
Mountain glaciers move down the valleys, scouring their 
bottoms and sides, and transporting much rock material. 

Denudation. — The combined action of these forces of 
weathering and erosion is denudation. In intimate relation 
they all act toward the one end of reducing the land ; and 
in this respect they are in opposition to the great internal 
force which is causing the land to rise and fall. They owe 
their power mainly to forces from without the earth. The 
moon and sun produce the tides, the sun causes the changes 
in the weather, the atmosphere acts as the intermediary, the 
ocean furnishes the water, and two internal forces furnish 
the opportunity, — internal heat and gravity. The former 
gives elevations to be destroyed, the latter draws the water 
to the earth and causes a tendency for the materials to move 
from higher to lower places. Weathering is the great agency 
of preparation; for their chief work, the erosive agents do 
some destruction and much transportation; and the ocean, 
aside from its work of erosion, is the great receiving ground 
for the waste from the land. 

These changes are in progress at all times, and they have 
been so through all of the geological ages, with the result 
that, although slowly acting, they have produced enormous 
changes. The present land forms are the result of the action 
of these forces (Plate 21 illustrates exceptionally rapid 
denudation) ; and since they are still acting as in the past, 
the surface of the earth is even now changing. The land is 
therefore in one stage of its history, and we must not look 



248 PHYSICAL GEOGRAPHY. 

upon the hills and valleys as unchanging and unchangeable, 
but rather as things of life, with a past history to be read, 
and a future to be predicted. 



REFERENCE BOOKS. 

Aside from those to which reference has been made at the close of the 
preceding chapter : — 
Lyell. — Principles of Geology, Vols. I. and II. Appleton & Co. , New York. 

Eleventh edition, 1872. 8vo. -$8.00. (This is the great geological classic, 

especially complete on the subject of denudation.) 
Shaler. — First Book in Geology. Heath & Co., Boston, 1884. 12mo. 

$1.00. (This interesting little book is written for beginners.) 
Shaler. —Aspects of the Earth. Scribner, New York, 1889. 8vo. $2.50. 

(Several chapters on topics touched upon in this and the preceding 

chapter.) 

For Soils, see article by Shaler, " Twelfth Annual Report IT. S. Geological 
Survey." Washington, D.C., 1891. 

For Artesian Wells, see article by Chamberlin in the Fifth Annual 
Report of the same, 1885. 

For importance of Earthworms, see Darwin, "The Formation of Vege- 
table Mould." Appleton & Co. (International Scientific Series), New York, 
1883. 12mo. $1.50. 

One of the most important contributions to denudation is Gilbert's 
"Geology of the Henry Mountains," Washington, 1887. (To be obtained 
only from the second-hand bookstores.) 

Many valuable and interesting papers appear in the regular geological 
periodicals, of which there are three issued in this country, as follows : 
(1) "Bulletin of the Geological Society of America." Six volumes already 
issued at $5.00 (to libraries) a volume. Address Professor H. L. Fairchild, 
Rochester, New York. (2) "American Geologist," Minneapolis, Minnesota, 
now in its sixteenth volume, two being published each year. Price $3.50 a 
year. (3) "Journal of Geology," Chicago, Illinois, now in its third volume. 
Price, $3.00 a volume. 



CHAPTER XIV. 

TOPOGRAPHIC FEATURES OF THE EARTH'S SURFACE. 

Continents and Ocean Basins. — The surface of the earth is. 
broken by a series of great irregularities, forming the conti- 
nents and the ocean basins. There are two groups of con- 
tinents, with intermediate basins filled with water. The 
continent masses, which may be called the eastern and the 
western, are mainly grouped about the north pole, causing 
the northern to be the land hemisphere ; and the oceans are 
gathered around the south pole, entirely surrounding it, and 
extending rather triangular tongues northward, toward the 
north pole. The two sets of continents are themselves 
more or less completely divided along nearly east and west 
lines. This division is north of the equator, and it is the 
cause for the separation of North and South America, and of 
Europe and Africa. With these partial or complete oceanic 
separations we have four great continent masses: North 
America, South America, Africa, and Eurasia. Australia, 
the fifth, is somewhat aberrant. 

The oceans are developed into two great basins, the At- 
lantic and the Pacific, the latter having an area of fully 
62,000,000 square miles, which is equal to nearly one-third of 
the area of the earth's surface. Besides these, there are the 
Arctic, Antarctic, and Indian oceans, which are only partially 
separated from the others. The Atlantic has an average 
breadth of a little less than 3000 miles, while the breadth of 
the Pacific is fully twice as great as this. And we find the 

249 



250 PHYSICAL GEOGRAPHY. 

same difference in size between the eastern and the western 
group of continents. The American continents have an 
average breadth of but little more than 2000 miles, while 
the average breadth of Europe and Asia combined, is over 
6000 miles. 

As has been described in Chapter IX., the oceans for the 
most part consist of great submarine plains or plateaus, here 
and there broken by gently rising ridges, or occasionally by 
steeply rising volcanoes or sharp mountain ridges. The pre- 



Fig. 127. 

Relief map of Eurasia (Lambert's projection). 

vailing feature of the ocean bottom is that of uniform level- 
ness ; and the average depth of this great submarine plateau 
is nearly three miles, while in some places the depth is over 
five miles. This great area, which is about three-fourths that 
of the earth's surface, is rendered level by means of the oceanic 
water which fills the basin. Above the ocean surface the 
continents rise with considerable uniformity, but their aver- 
age elevation is very much less than the depth of the ocean. 
The average elevation of the land surface of the globe is 



TOPOGBAPHIC FEATURES OF EARTH'S SURFACE. 251 





Pacific 



Colorado 



Miss.R. 



Appalach- 
ians 

Atlantic 



about 2000 feet ; and it is only 
here and there, along mountain 
chains and plateaus, that greater 
elevations are found; but the 
average depth of the ocean is 
fully six times as much. This 

difference between land eleva- g 

tion and ocean depression, is I 

shown in Fig. 128, which repre- | 

sents a cross-section of North £ 

America and the Atlantic © 

Ocean, drawn upon the same j* 

vertical scale, which is greatly § 

exaggerated. S" 

Examining the continents in g 

a little more detail, we find that g; \ 

they consist of plateaus and jj> E 

plains as the most prominent g 

features. Usually there are 5' 

two plateau areas, one upon g 

either margin of the continent ; £ 

and above these rise more or rt 

(3* 

less continuous ridges, which £ 
we know as mountain chains. £ 
This feature of continents is 8 
well illustrated in North Amer- sr 
ica (Fig. 129), which may be 
considered a typical continent ; 
and it will be described in more 
detail in later parts of the chap- 
ter. Plains usually occupy the S 
interior portion of the continent, ^ 
and these are sometimes in the form of low plateaus ; while 



Mid 

Atlantic 
Bidge 



Spain 



252 PHYSICAL GEOGRAPHY. 

in some cases they are even interior basins. The land surface 
is very irregular, the irregularities being partly due to origi- 
nal features of the earth's crust, and partly to the sculpturing 
of these by the agents of denudation (see Chapter XIII.). 
Geological conditions conclusively prove that the con- 



Fig. 129. 
Relief map of North America (Lambert's projection). 

tinents are subject to changes, and that the present form is 
merely the result of an evolution which has long been in 
progress. Even at present, in some cases, there are changes 
of considerable moment still in progress. The mountains 
which form the border of the continents have been elevated 




Face page 253 






TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 253 

by successive foldings of the rocks ; the plateaus have been 
produced by great elevations of the land ; and many of the 
plains have been caused by the filling of seas from the waste 
of the mountains. Many forces have cooperated to build the 
continents, but this subject is properly one of pure geology. 
From the present standpoint, it is enough to know that these 
changes are going on, and to recognize the fact that the 
continents are not of the same form at all times. Indeed, 
there seems good reason for believing that the true Ameri- 
can continent does not end at the present shore line, but 
that its proper boundary is along the margin of the conti- 
nental shelf, which on the northeastern side, extends to a 
distance of from 50 to 100 miles from the present shore. 
At this point the great ocean abysses commence, and from 
the land to this point there is very little depth to the ocean. 
(See Fig. 128.) 

Physical Geography of the United States. — The best way 
to illustrate the typical features of the earth's surface, is not 
to make a hasty survey of the entire surface, but rather to 
consider a single area in some detail. Thus we may select 
the United States (Plate 22) as a typical part of the con- 
tinent, and by examining the physical geography of this area, 
form an idea concerning the main features of the earth's sur- 
face ; for we have very nearly every important topographic 
form represented within the boundaries of this country. For 
the sake of completeness, it will be well to extend the boun- 
daries of the area described, for a short distance beyond the 
Canadian boundary, in order to include a portion of the con- 
tinent which is essential to its proper consideration. This 
area, including the United States and southern Canada, 
forms a true section of a typical continent. We may divide 
this area into five great divisions, each having characteristic 
geographic features. These are the Atlantic Coast Province, 



254 PHYSICAL GEOGRAPHY. 

the Eastern Mountain Ranges, the Canadian Highlands, the 
Mississippi Valley Plains, and the Cordilleras of the West. 

Atlantic Coast Area. — This properly includes the conti- 
nental shelf, which is now submerged beneath the sea, but 
which is a submarine plain bordering the continent and 
appearing to form a true part of it. Above the sea level, 
the continuation of this area is represented by a narrow strip 
of level country with an elevation of but a few feet above sea 
level, and extending from New Jersey to the Rio Grande. 
It forms a low plain which is nearly featureless, and which 
in some parts is in the condition of a swamp. It is but a few 
miles in width in the northern portion, and varies in width 
as we proceed southward, but gradually increases until the 
Gulf States are reached. A large part of Florida is included 
within the area, and along the Mississippi valley the coastal 
plains expand and extend inland to a considerable distance. 
The present delta and floodplains of Mississippi, Louisiana, 
and a part of Arkansas, belong to this coastal area ; and in 
Texas there is a strip whose width is often as great as 50 miles. 

On the landward side of this low-lying plain, is a more 
elevated area of level country, which is also a true plain, but 
which is more ancient in origin. The low swampy plains 
are scarcely drained ; but these higher, inland plains, are cut 
by river valleys, and in some cases carved into a series of 
rounded hills. For the most part, the low swampy plains, 
near the coast-line are of little use to man, their swampiness 
prohibiting their occupation, though this does not apply to 
some parts of the plains, such as the delta and floodplain 
region of the Mississippi. The higher plains on the land- 
ward side of these, are much better adapted to occupation, 
and it is upon these that the greater part of the agriculture 
of the Southern and Gulf States is carried on. 

The Eastern Mountains. — In nearly all cases mountain 



TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 255 

chains are found rising above basal plateaus. This is true 
for the great system of eastern mountains, the Appalachians. 
Both on the eastern and western sides of these chains, there 
is a highland country which is a true plateau, though in 
most cases deeply carved by stream valleys. There are two 
parts to this system of eastern mountains, one much older 
than the other, and both considerably destroyed by the 
sculpturing action of the agents of denudation. The oldest 
series of mountains date back to the first beginning of the 
known history of the North American continent, when they 
Were formed as very high mountain chains. A considerable 
part of New England is included within this area of ancient 
mountains, and the chains extend southward through the 
hills of New Jersey, and thence along the eastern base of the 
modern Appalachians into the Carolinas. In many places 
these would not be recognized as mountains, but are now in 
the form of low hills. They have been worn down to their 
very roots, and nothing but hills are left where once existed 
very lofty chains. The highest remnants of these mountains 
are found in New England and North Carolina. 

The true Appalachians were much more recently formed; 
but yet they are among the ancient mountains of the conti- 
nent. For a long period of time they also have been exposed 
to the destructive action of denudation, so that their original 
form is very much altered. They are no longer high chains, 
and in point of size and grandeur bear no comparison with 
such recent mountains as the Rockies, the Andes, or the 
Alps. Formerly they were much higher than now, and 
probably their features were much more like those of the 
grander mountains of the globe. At present they consist of 
a series of ridges and ranges, extending in a northeasterly 
direction, usually with nearly level tops, and in no case rising 
to great heights. 



256 PHYSICAL GEOGRAPHY. 

The highest part of the eastern mountains is Mitchell's 
Peak in North Carolina, whose elevation is 6688 feet. In 
these mountains there are vast stores of coal, building stone, 
iron, and other products which are of use to man. 

The Canadian Highlands. — These are another ancient 
series of mountains, once much more extensive than now, 
and they enter this country in only one or two places. The 
Adirondacks may be considered a part of this highland area, 
and the same holds true for the hilly region near Lake 
Superior. At present this region is occupied by a series of 
low, rather rounded hills, never rising to great mountain 
heights, and rarely being over a mile above the level of 
the sea. Among the Adirondacks the highest point is Mt. 
Marcy, which is 5379 feet above sea level. For the most 
part this hilly region is of little value, partly because it is 
situated far in the north, and partly because it is composed 
of rocks that do not favor the formation of even slopes and 
deep soil. There are considerable areas of valuable mineral 
materials, mainly iron and copper. The St. Lawrence valley 
forms quite another province. 

The Central Plains. — Extending from the western base 
of- the Appalachians to the Mississippi, there is a great 
area of plains, which gradually decrease in elevation toward 
this river. From the Mississippi westward, the plains con- 
tinue until the base of the Rocky Mountains is reached ; 
and here also, as the mountains are neared, the elevation 
gradually becomes higher. At the base of each of these 
mountain systems, the plains have become transformed to 
true plateaus, in the case of the Appalachian plateau with 
an elevation of 1000 or 2000 feet, and of the Rocky Moun- 
tains with an elevation of over 5000 feet. 

This great area of plains is not everywhere level or roll- 
ing, but in some of its parts is broken by truly moun- 



TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 257 

tainous irregularities. This is true, for instance, in Indian 
Territory, in Arkansas, in part of Missouri, and elsewhere. 
Aside from these limited areas of mountainous character, 
there are other regions which have been very much cut and 
dissected by stream action. However, the general condition 
of these plains is that of gently undulating country. They 
form the great farming belt of the continent, and also 
contain deposits of valuable minerals, such as coal, iron, 
petroleum, building stones, etc. This area of plains is 
equal to fully one-fourth of the total area of the country, 
and the elevation is generally less than 2000 feet, while 
nearly one-half of the area has an elevation of less than 
1000 feet. 

Since occupied by man, the greater part of this area has 
been free from timber. In the plains of the far west this 
is due to the fact that the climate is dry; but among the 
prairies of the east the cause is less easily ascertained. Some 
think that, because of its compactness, the soil was unfavor- 
able, others that the timber has been burned off by fires ; 
but neither theory can be considered proven. 

The Cordilleran Area. — This is the most complex of our 
geographical areas, and perhaps should be subdivided, though 
for our general purpose it may be considered as one great 
area. In the main it consists of a great plateau, with an 
average elevation of over a mile above the sea level, above 
which rise several mountain chains. Commencing on the 
eastern base of this Cordilleran region, we will examine it 
in cross-section until the Pacific is reached. 

A high plateau reaches to the very base of the Rocky 
Mountains, which then rise to great elevations, not only 
above the sea level, but also above the plateau itself. The 
highest part of the Rockies is in Colorado, in which state 
there is a total area of nearly 13,000 square miles with an 



258 PHYSICAL GEOGRAPHY. 

elevation greater than 10,000 feet, while several peaks rise 
above 14,000 feet. The chains, which extend northward 
and southward, are of varying heights and differ also in 
extension. There is not one mountain chain, but a series 
which together make the Rocky Mountains. They pass 
entirely across the United States, entering Canada on the 
north and Mexico on the south. 

West of these mountains is a region of interior drainage, 
known as the Great Basin. In reality there are numerous 
interior basins, some of which combine to form a Great 
Basin (Plate 23 and Fig. 151), while others exist as separate 
smaller basins of interior drainage. The basin region is a 
great plateau area, generally above sea level, and usually 
more than a mile above the level of the sea. It is commonly 
surrounded by high mountains, and the interior plateau itself 
is broken by ridges, known as the Basin Ranges, which 
extend in a north and south direction. 

Bordering the Great Basin on the west, is the Sierra 
Nevada range, which passes in a nearly north and south 
direction, from the northern part of California to the southern 
border of the country. 1 It is a high mountain region, but 
its average elevation is less than that of the Rockies. 

West of the Sierras is a great valley, which, with minor 
interruptions, extends from Canada to Southern California, 
where it is interrupted by a mountain mass, to the southeast 
of which the broad valley continues into the Great Basin, to 
the Gulf of California, which is really a part of this valley. 
Death Valley of Southern California, which is a part of the 
Great Basin, is an illustration of the rather rare feature of 
an interior basin below sea level. It is 175 miles long, and 
in one place is at least 225 feet below the level of the sea. 

1 There is no uniformity in the usage of the term Sierra Nevada, and the 
boundaries of the range are vaguely and variously drawn. 



TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 259 

West of this valley, and rising almost out of the Pacific, 
is a fourth series of mountains, the Coast Ranges, which 
extend from Lower California to the northern boundary of 
the United States, and apparently as far as Alaska. They 
are rugged mountains, and among them are found some of 
the highest peaks on the continent. 

These mountains of the Cordilleras are much more recent 
than those of the eastern part of the continent. Many of 
them were formed in the Tertiary period, and there is evi- 
dence that some of them are still growing. It is as a result 
of this that they are so rugged and so high ; for they have 
not been long enough exposed to the action of denudation to 
be reduced to low, rounded forms. 

No mention has been made of volcanoes, for the reason 
that within the borders of the United States, outside of 
Alaska, there are none known to be active. That this has 
not always been the case, is shown by the vast number of 
volcanic cones, in all stages of destruction, which dot the Cor- 
dilleran region. There are thousands of these (see Chapter 
XX.); and on every hand, the evidence is conclusive that in 
very recent times large areas have been deluged in lava and 
ash deposits. Along the eastern margin of the country there 
is no sign of recent volcanic activity, although during the time 
of formation of the higher mountains, volcanoes did exist. 

Aside from being the largest geographic zone of the 
country, the Cordilleran region contains minerals in extraor- 
dinary variety and abundance. It is the great precious 
metal zone of the earth, and from it is produced more gold 
and more silver than is supplied by any other region. 

The Drainage of the Country. — Three oceans receive the 
waters that fall in the United States. The accompanying 
map (Plate 23) shows this so graphically that description 
may be omitted. 



TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 261 

The Shore Line. — The coast of North America, like that 
of several other continents, encloses a land area which is 
triangular in form, the apex of the triangle being toward 
the south (Fig. 129). There is much variety in the form 
of the coast ; but the coast line is by no means so broken 
as that of Europe (Fig. 127). There are several great 
promontories, notably on the east coast, and many minor 
irregularities along this coast line ; and these are much 
more prominent in the northern than in the southern part 
(Plate 22). In the east, islands abound along the coast of 
Maine, and near Florida. On the Pacific coast there are 
very few islands, excepting those which begin to be numer- 
ous at the northern boundary of the United States, and in- 
crease in abundance toward Alaska. 



REFERENCE BOOKS. 

Whitney. — The United States. Little, Brown, & Co., Boston, 1889. 
8vo. $3.00. (In the main reproduced from an article by the author in 
the Encyclopedia Britannica.) 

Shaler. — The United States of America. Appleton & Co., New York, 
1894. Two volumes. 8vo. $10.00. (Particularly Vol. I, in which the 
relation between man and nature is pointed out.) 

Shaler. — The Story of Our Continent. Ginn & Co., Boston, 1891. 
12mo. $0.75. (Of value to the student for elementary reading.) 

As a type of the kind of book needed to adequately describe the 
features of this country, attention may be called to Geikie, — Scenery of 
Scotland. Macmillan & Co., New York, 1887. Second edition. 8vo. $3.50. 

For known elevations of points in the United States, see Gannett. — 
Dictionary of Altitudes, etc. Bulletin 5, U. S. Geological Survey, 
Washington, 1884. 8vo. $0.20. Second edition of same, revised, Bulletin 
76, 1891. 8vo. 393 pages. $0.25. 

For average elevation of the states and country, see Gannett, Thirteenth 
Annual Beport, U. S. Geological Survey, Washington, 1893. 



CHAPTER XV. 



RIVER VALLEYS. 



General Description. — A river is a natural drainage lme 

on the land, and it usu- 
ally occupies a valley 
which has certain quite 
definite characteristics. 
On either side of the 
river are the valley sides 
or walls, sometimes ris- 
ing gently, sometimes 
steeply : at times to 
great heights (Fig. 
130), and again with 
only low elevations. In 
the lowest part of the 
valley, and generally 
near its middle portion, 
the river flows, usually 
with a meandering 
course. In most cases 
the river is immediately 
bounded by rather 
steeply rising banks, only 
a few feet in elevation, 
beyond which the slope 
is more gentle, and 
sometimes even a plain, 
which in times of flood is covered with water (Fig. 157). 

262 




Fig. 13U. 
A deep mountain valley. 



RIVER VALLEYS. 



263 




Fig. 131. 
Stream issuing from a limestone cave. 



Here and there, at irregular intervals, tributaries enter the 
main stream, and these themselves branch into other tribu- 
taries, until very often there is finally produced a branching 
network of minor streams, all directly or indirectly contribut- 
ing to the supply of water in the main stream. This princi- 
pal supply often comes 
from springs, and some- 
times the source of the 
river is a spring (Fig. 
131). Together these form 
a river system; and such a 
system may have an area 
of only a few miles, or it 
may drain an area of 
many thousand square 
miles. The line or plane 

which separates one stream or system of streams from 
another, is known as a divide or water parting. 

All streams have these general characteristics ; but when 
their valleys are examined in detail, there are found to be 
many differences, not merely between different rivers, but 
even in the several parts of the same river. The valley of 
one stream, or a part of a stream, may be a precipitous gorge 
(Fig. 142), or it may have very gently sloping sides (Fig. 132). 
In some rivers there are floodplains and deltas, in others 
these are absent ; and in some cases the rock walls of the 
valley may rise directly out of the river. In some there 
is a permanent flow of water, in others the supply is inter- 
mittent, and in some extreme cases water flows only once 
or twice a year. In most river systems the tributaries are 
numerous, but in some cases they are few ; and while some 
of these join the main streams at a high angle, in many cases 
they enter it at an acute angle. There are many reasons for 



264 



PHYSICAL GEOGRAPHY. 



these differences in streams, the two most important being 
the position and kind of rock in which they occur, and the 
stage in development which they have reached. 

Only a very few years ago, river valleys were believed to 
have been formed by some earth convulsion, or some un- 
usual force, and it was thought that rivers occupied them 




Fig. 132. 

Brink of Niagara Falls. A valley having very gently sloping sides above the 

falls and precipitous sides below. 

merely because they were valleys ready made. It was 
believed that the crust of the earth had been contorted and 
fissured, that ocean floods had swept over the land, and that 
the rivers had practically no share in the formation of the 
valleys which they occupied. We now know that the major- 
ity of rivers have formed their own valleys, that they have 
formed them in a very slow way, and that most of them are 



RIVER VALLEYS. 



265 



still engaged in 
the work of val- 
ley carving. If 
this be so, then 
river valleys 
have had a de- 
velopment and 
a history ; and 
in this history 
we may hope to 
find an explana- 
tion of many of 
the differences. 
Development 
of River Val- 
leys. — We must 
bear in mind 





Fig. 134. 

Royal Gorge, Col. A mountain gorge 

where erosion is in rapid progress. 



Fig. 133. A gorge near Ithaca, N.Y., illustrating the 
down-cutting of a stream valley and its broadening 
by weathering. 

that the river valley is the 
result of the combined action 
of stream erosion, which tends 
to deepen (Fig. 133), weath- 
ering which tends to broaden 
the valleys (Fig. 123), and the 
transportation of sediment fur- 
nished by these means. Ero- 
sion proceeds more rapidly 
than weathering, but there 
comes a time when its action 
is checked. In no part of the 
valley can the stream cut be- 
low the sea level, or below 
base level, as it is called ; and 
since it must carry water 



266 PHYSICAL GEOGBAPHY. 

down a slope, in its erosion the river reaches lower levels 
near its mouth than higher up in its course. Until a 
line of easy slope is reached, erosion exceeds weathering 
(Fig. 134) ; but then, since erosion is checked while weath- 
ering continues, the latter produces its most marked effect, 
and the valley gradually broadens, while the hills slowly melt 



Fig. 135. 

Oxbow cut-off in Connecticut Valley, Northampton, Mass. A broad matuiv. 

river valley with rounded valley walls. 

down. Unless interfered with, this would continue until the 
surface was base-leveled, or reduced to a nearly level con- 
dition. 

Different parts of the river will work at different rates ; 
and under variable conditions, this development from the 
canon or gorge-like valley of youth, to the broad valley with 
rounded sides (Fig. 135), which characterizes the more 



RIVER VALLEYS. 



267 




mature stages, may proceed at different rates and with, dif- 
ferent results. Naturally the part of the river which is 
nearest the mouth 
is first and most 
easily developed, 
for it is nearest the 
sea level. There- 
fore a stream val- 
ley may have the 
narrow gorge-like 
condition among 
its head waters, 
while the lower portion is broadened into mature form. 

A part of the stream 
may flow through soft 
rocks, and another por- 
tion through hard lay- 
ers ; and then, even 
in short distances, the 
form of the valley may 
change ; for the process 
of broadening by weathering is much more rapid in the soft 
than in the hard strata 



Fig. 136. 

Development of the canon. A and C, layers of hard 

rock, B and D soft. 




Fig. 137. 

Development of the canon profile in an arid 

region. 1 and 3 hard, 2 and 4 soft strata. 



, \ \ 



K 






//, 



■AU—4-i 






-.&.&■'■ 



A stream may be able to 
cut in one part of its course, 
and be obliged to deposit 
sediment in another por- 
tion ; and in the latter 
case, erosion is checked, 
while weathering continues 
to produce a perceptible 
effect. In an arid climate, where weathering is relatively 
ujiimportant, the valleys are almost all in the condition of 



Fig. 138. 
Diagrammatic representation of develop- 
ment of a young valley (aa) to old 
age (hh). 



268 



PHYSICAL GEOGRAPHY. 



the gorge or canon (Figs. 136, 137) ; and among moun- 
tains, where the elevation and slope are great, erosion so 
exceeds weathering that the gorge is the characteristic 
valley (Fig. 134). 

During the time when erosion exceeds weathering, — that 
is during youth, — the resulting valley is deep and relatively 
narrow ; and wherever we see this kind of valley, we may 




Fig. 139. 

The Yellowstone, a young valley broadening by weathering and being deepened 

along a narrow line by the river erosion. 



be certain that, for one reason or another, erosion is now, or 
has recently been in progress. That weathering is also pro- 
ducing an effect, is evident from the fact that the valley is 
wider at the top than at the bottom, because the former has 
for a longer time been exposed to its action (Fig. 139). In 
such cases the river is often a series of cascades or falls, 
because (see Chapter XVI.) in its rapid down-cutting, the 



RIVER VALLEYS. 



269 




Fig. 140. 

A bit of drainage in Illinois, showing slight 
development of tributaries. 



stream finds rocks of different powers of resistance, and 
therefore cuts its bed irregularly. Therefore, in addition to 
gorges, waterfalls characterize youthfulness in river valleys. 
In many cases lakes are also present ; and since the process 
of lake destruction or re- 
moval is a simple and brief 
task, they do not long re- 
main in the river valley. 

The development of the 
stream proceeds most rap- 
idly near its mouth, and 
later in the headwaters ; 
and consequently, tribu- 
taries are not numerous 
at first (Fig. 140) ; but 
one by one they begin to 
develop, until all of the area is brought under the influence 
of some stream or rill (Fig. 141). At first the divides are 

not very definite, and they may 
be flat-topped and swampy ; but 
in maturity these become quite 
sharply defined, and usually every 
part of the area is drained. 

When vertical erosion has 
ceased, the work of the river be- 
comes merely that of a transporter 
of sediment, except that in swing- 
ing about, the river does some 
lateral erosion on its banks. The 
characteristics of youth disappear, 
waterfalls are worn down, lakes 
are filled and destroyed, the gorge is broadened to the gently 
sloping valley side (Fig. 138), and the number of tributaries 




Fig. 141. 

A bit of West Virginia drainage, 
illustrating well - developed 
tributaries of maturity. 



270 



PHYSICAL GEOGRAPHY. 



increases. With the broadening valley, and the decrease in 
river slope, the conditions favoring noodplains are brought 
about ; and since the first and most rapid development is in 
the lower part of the river, in this stage the valley may con- 
sist of three quite different parts, — a lower flood-plained 
course, a middle portion, and an upper torrential part, with 

gorges and waterfalls. The 
l|| majority of streams have 
reached this stage, and 
this is why, in describing 
a river, it is commonly said 
that it consists of these 
three parts ; but really 
this is to be considered as 
merely a stage in develop- 
ment, to reach which other 
stages are passed through, 
and which is normally suc- 
ceeded by others. Since 
all rivers are not in the 
same stage of development, 
a careful examination of 
the valleys of a country 
shows many exceptions to 
this condition of early ma- 
turity. 
Naturally there is much difference in the rate of develop- 
ment, and in the result produced under different circum- 
stances. Whether the river develops in a mountain or on a 
plain, or in an arid or humid climate, the main fact is the 
same, — that there is this development from immature gorge 
to broad valley. On a low plain near the sea level, the rate 
of development in the soft clay is much more rapid than in 




Fig. 142. 
Canon of the Colorado. 



RIVER VALLEYS. 



271 



a high plateau ; but while in the former there are produced 
only shallow trenches a few feet in depth, in the latter a 
canon may be cut with a depth of thousands of feet. The 
former we see in the plains bordering the coast of Texas, 
the latter in the Colorado canon (Fig. 142). In the hard 
rocks of the Colorado the form of the canon is preserved, 
and this is also favored by the dry climate ; but the soft, 
clay banks of the _ _... 



Texas streams 

readily crumble 

under the action 

of weathering in a 

moist climate. The 

development of the 

latter to the state 

of maturity, will 

therefore be much 

more rapid than 

that of the former, 

— just as some 

animals or plants 

pass through life 

in a few weeks, 

while others live 

for a century. 

In a mountainous country the elevation is so great, and 

the rock structure so complex, that gorges will remain for 

long periods of time ; and ages must elapse before the 

erosive action of the river becomes less rapid than weather- 
ng. Now and then a deep mountain lake may check the 
vork of the river, and serve as a temporary base level, below 
fhich, for the time being, the stream cannot cut ; and so 
lere, for a short distance, the valley may become broadened. 




Fig. 143. 

A broad Alpine valley. 



272 



PHYSICAL GEOGRAPHY. 



While the prevailing type of mountain stream valley is 
that of the gorge (Fig. 134), there are mountain valleys of 
great breadth and depth. These are not true stream valleys, 
but great synclinal valleys of rock folding (Fig. 143) which 
the rivers have occupied because of their convenient location. 
After passing through a deep defile (Fig. 144), a tiny stream 
may emerge into one of these great, park-like valleys (Figs. 

143 and 221); and 
then we see, side 
by side, the valley 
of stream forma- 
tion and that of 
rock folding. 
With the aid of 
weathering, even 
the mountain gorge 
will in time broad- 
en out into a wide 
valley. 

Adjustment of 
Streams. — When 
a river begins to 
cut its valley upon 
a new land, there 
is no necessary 
relation between 
stream course and rock structure. The stream may flow 
across hard and soft layers alike, the course being consequent 
on the topography, because the river was guided down the 
original slopes. However, as the river develops, it often 
gradually changes its course in order to follow soft la3 r ers 
of rock; and therefore, in regions where the rock layers 
are inclined, many river courses are adjusted to the rock 




Fig. 144. 
Mountain gorge in the Alps. 



RIVER VALLEYS. 



273 



structure, soft layers being the site of valleys, while the hard 
strata stand out as ridges. This is characteristic of mature 
streams which have had a long period of development and 
change. 

At first the topography guides the stream course, but 
finally the river course determines the topography. In such 
regions as New England, we find the large river valleys 
cut in the softer beds of rock, while the harder strata stand 
up as ridges. Still, here and elsewhere, there are numerous 
exceptions to this statement, which is only generally true. 
This mature adjustment is well shown in many of the New 
England and Appalachian streams. Some of the ways in 
which these changes take place are described in the next 
section. 

The River Divide. — Between any two streams there is a 
line, or an area, which divides the waters, sending a part one 
way and the rest in an opposite direction. These divides or 
water partings are by no means permanent, but are con- 
stantly and usually very slowly 
changing. The stream that has the 
most power pushes the divide into 
the territory of the other, and there 
are various ways in which one stream 
may have more power than another. 
One may have a shorter course to the 
same level, and hence have a greater 
slope (Fig. 145); or one may be 
cutting through soft rock, while the 
opponent is working in hard layers 
(Fig. 146); or (as in many islands in the trade-wind belt) 
the rainfall on one side of the divide may exceed that on the 
other. Gradually the divide moves into the area of the stream 
laving the least rainfall, or the least slope, or the hardest rock. 




■"H 



v ^ 



Fig. 145. 



274 



PHYSICAL GEOGRAPHY. 




Fig. 146. 



A still more important cause for the change of divides is 
found among tilted rocks. If the layers of a series of strata 
stand in the monoclinal attitude, and if these alternate in 

hardness, the soft layers 
weather more rapidly 
than those which are 
hard, and which, because 
of this fact, tend to re- 
main above the general 
level (Fig. 261). In 
such a case, the highest 
points do not sink verti- 
cally as the ridges wear 
down ; but they move 
downward and back- 
ward in the direction 
of the dip of the strata 
(Fig. 147). This is so permanent a condition that it may 
be stated as a law, that in rocks of monoclinal attitude the 
divide migrates in the direction of the dip. This law of 
monoclinal shifting applies also to changes in river courses. 
In their down-cutting, the valleys also tend to migrate in 
that direction, and this is one of the reasons why streams 
adjust themselves 

c 



to soft layers ; for 
once finding them, 
they tend to re- 
main in them. 

Usually the mi- 
gration of a divide is an extremely slow process, and in 
the course of a lifetime one would not notice any change ; 
but under exceptional circumstances it may become rapid, 
and in a brief time the divide may change for many miles. 




Fig. 147. 
Illustrating monoclinal shifting of divides. 



BIVER VALLEYS. 



275 




This will happen when a river with a more favorable situa- 
tion, for some reason gradually pushes its divide back until 
it taps its opponent. Then the stream receives a large acces- 
sion of drainage area and carries a part of another system 
across the old divide (Fig. 148). Before the diversion, the 
divide was low and 
nearly on the same 
level as the stream 
about to have its 
course changed ; and 
then, perhaps during 
some time of flood, 
the new course was 
chosen and after- 
wards maintained. 
While these cases 
undoubtedly occur, 
it is doubtful if they 
are at all common ; and the ordinary change in the divide is 
a very slow one. By these changes in divides, the adjust- 
ment of streams is also favored. 

Accidents to Streams. — River valleys tend to pass through 
a regular cycle of development, from the young to the old 
stages ; and if nothing intervened to prevent, we should find 
them all in some stage in this regular cycle. Some would 
be young, others mature, and others old ; some would be 
upon plains,^others on plateaus, or among mountains. There 
svould be great variety in river valleys, but it would be of 
. regular kind. In reality, the development of rivers is 
ubject to many interruptions of various kinds, and the cycle 
s never entirely passed through by any single river. The 
ccidents to which rivers are subjected, sometimes increase, 
ometimes decrease, the power of the stream. In the course 



Illustrating sudden shifting of a divide (aa) to 
(bb) by carrying the headwater (e) across the 
old divide at (c). 



J 



276 



PHYSICAL GEOGRAPHY. 



of its development, the different parts of a river may experi- 
ence entirely different accidents, and the resulting valley 
will be complex or composite. Any single part of a stream 
may also suffer a variety of accidents. 

Land Movements. — Land movements are among the most 
common accidents which interfere with normal development ; 
and these are of three kinds : (1) broad uplifts, (2) down- 
ward movements, (3) folding which accompanies mountain 
formation. With the general uplift of a country, streams 
are given new life, or rejuvenated, and we may then have 
a narrow gorge cut in the center of a broad valley. After a 
long period of denudation the uplift gives new powers to 

the stream, and it 
then cuts a nar- 
row valley (Fig. 
149). Such an 
uplift may affect 
great areas; and 
in the rivers thus 
revived, waterfalls 
again begin to develop, and nearly all of the appearances 
of youth may return. Nearly every stream system shows 
some sign of this kind of rejuvenation, which has affected its 
recent development. If this elevation happens near the sea- 
coast, a part of the ocean bottom is raised to the condition 
of dry land, and the streams of the old mainland extend 
across it ; and perhaps by this means separate streams may 
be united to form one system. 

Depression of the land would rob streams of some of their 
force by decreasing their elevation, and hence their slope. 
Along the coast, the lowering of the land causes the ocean 
to extend up the valleys, drowning parts of the streams, and 
transforming their mouths to estuaries or straits, while 




Fig. 149. 
Diagram showing the result of an elevation, which 
caused the inner canon of the Colorado to he cut be- 
tween the older walls of the outer and broader valley. 



RIVER VALLEYS. 



277 




Plate 24. 
River valleys drowned by submergence beneath the sea. 



278 PHYSICAL GEOGRAPHY. 

numerous islands are formed where the hilltops rise above 
the sea (Figs. 193, 211 and Plate 24). This entrance of the 
sea produces a reverse effect from that of elevation ; for 
the lower parts of streams may be dissected, and parts of one 
system may enter the ocean through separate mouths. This 
is very well illustrated in many cases on the coast of Maine, 
and particularly well in the Chesapeake (Plate 24), which, 
with its tributary streams, represents a part of a river system 
drowned by the sea. 

When the strata are folded in the form of mountains, 
stream erosion is interfered with and often entirely checked. 
As the mountains rise, a dam is built in the path of the 
rivers ; and unless their rate of down-cutting is as rapid as 
the rate of elevation, which in most cases would not be true, 
the streams will suffer interruptions. If they persist in 
their course, and cut their channels as rapidly as the moun- 
tains rise, they are known as antecedent streams. It is 
doubtful if there are many cases of rivers now crossing a 
large mountain in exactly the same course which they occu- 
pied at a time antecedent to the mountain formation ; but 
many geologists believe that the Green River, where it 
crosses the Uinta Mountains of Utah, is an illustration of 
this type of stream. 

Ordinarily the folding would locally transform the river to 
a lake, and as the dam continued to grow, the lake would 
gradually become deeper and more extensive. With the 
formation of the lake the erosive power of the stream 
decreases ; for when it flows from the body of quiet water, 
it has been robbed of its sediment supply, and is therefore 
unable to do much erosive work. If the mountain growth 
is rapid, it may even cause a stream to flow in a direction 
opposite to the course which it originally had — it maybe 
diverted or even inverted. Where the rocks in the middle 



BIVER VALLEYS. 279 

course of a river are rapidly folded during mountain growth, 
a stream may even be separated into two parts. 

With the growth of the mountain, since the river slope is 
increased, new tasks are set before the streams. Gorges and 
waterfalls are caused, and because of the great elevation of 
the mountains, these continue for a long time ; and thus long- 
continued youth is impressed upon the mountain valleys. 
Every mountain furnishes illustrations of these latter feat- 
ures ; and in many, such as the Alps, there are also lakes, 
which are the result of mountain folding, and which repre- 
sent the interference with stream erosion which is brought 
about by the growth of mountain dams. 

Climatic Accidents. — A change of climate to a condition 
of dryness, robs streams of their erosive power ; but even 
more markedly does it decrease the power of weathering. 
Hence such a change favors the angular type of valley. It 
reduces the number of streams (Fig 150), and causes those 
which remain, to be dry for a large part of the time ; and 
hence in a dry country, there are large areas unoccupied by 
drainage lines. A rare, heavy rain, falling upon such an un- 
drained surface, carves a temporary valley, or arroya, which 
may never again be occupied by water. Stream valleys may 
be permanently abandoned, while others may be only withered 
or shrunken. By the increasing dryness of the climate, lakes 
may be evaporated and great basins of interior drainage be 
formed. Therefore, stream systems may be dissected by this 
cause also, and channels of outflow of lakes may be aban- 
doned (Fig. 151), while the direction of the drainage changes 
from the sea to the lowest point of the old lake bottom ; and 
this causes many other peculiar changes of a minor nature. 
By this action, a part of the Great Basin which was once 
tributary to the Pacific, through the Columbia, is now trans- 
formed to the Great Salt Lake interior drainage area. 



280 



PHYSICAL GEOGRAPHY. 



The change in climate which produces glaciation, first 
covers all the country with ice and buries the valleys. Near 
the margin of the snow-covered area, streams may be sepa- 
rated, and an entire change in the drainage be caused. 




Fig. 150. 
The drainage of an arid region 

Among the effects of the ice front, is the interference with 
streams that flow toward the ice, which acts as a dam, trans- 
forming them to lakes, and causing them to overflow across 
some divide. When the ice of the North American conti- 



RIVEB VALLEYS. 



281 



\*ru *r.!<; o^maqx 




s/s3 



/' W 




i '? h 



nental glacier (see Chapter XVII.) was melting from the 
surface of the country, many such lakes were produced, and 
some of them were of great size. The St. Lawrence system 
was dammed, and lakes were produced in different positions 
from those occupied by the present Great Lakes. During 
the same period, the valley of 
the Red River of the North 
was transformed to a great 
lake which overflowed to the 
Gulf of Mexico, instead of to 
the Arctic, as the present 
drainage directs. 

Some streams had their 
courses permanently changed 
and even reversed. When the 
ice melted, it left much drift 
material upon the surface ; and 
this soil sometimes completely 
buried the old valleys, so that 
entirely new channels had to 
be cut. More often this filling 
was only partial, and streams 
were turned from their course 
for short distances, and often 
dammed into lakes, which in 
many cases are now repre- 
sented by swamps (Plate 25). Hence in a glaciated region 
we may have very complex streams ; for in broad, mature 
valleys, local post-glacial gorges may be cut, while here and 
there falls and lakes exist. The streams are often given 
new life, or rejuvenated, either through their entire course, 
or for a short distance. Often the course forced upon them 
is very much more roundabout than that pursued before the 






Fig. 151. 

The Great Basin. The lighter shad- 
ing shows the former extension of 
lakes when the Great Salt Lake 
overflowed into the Columbia. 



282 



PHYSICAL GEOGRAPHY. 



glacial period (Fig. 152). Illustrations of the various effects 
of glaciation abound by the thousand in the glacier belt 

of New England, 
New York, and 
other of the 
Northern States. 
Nearly all of the 
gorges and lakes 



in this belt are 
the result of the 
condition of glaci- 
ation. While the 
most notable in- 
stances are those 
of the Great 
Lakes and Ni- 




Fig. 152. 

Diagram of a river caused to flow irregularly because 
of glacial deposits in its course, which prevented it 
from entering the main stream by its preglacial 
course, now partly occupied by a tiny stream. 



agara, these are merely large examjjles of a great group. 

Other Accidents. — Interference with river valley develop- 
ment is commonly noticed in regions of volcanic eruptions. 
Sometimes the valleys are filled with lava ; at times the 
streams are forced to cut new valleys in a part of their 
course ; again they are transformed to lakes ; and they may 
even be forced to flow in a reversed direction. Here again, 
the valleys are rejuvenated, and gorges and falls are pro- 
duced. Illustrations of these features may be seen on almost 
any map of a region of volcanic activity. 

An avalanche in a mountain may produce one or all of 
these effects, and there are other minor accidents to which 
streams are subjected. Sometimes river valleys are again 
and again subjected to one or several of these accidents, and 
their cycle of development much interfered with. This is 
why youth and early maturity are the characteristic features 
of most valleys ; for the stage of old age cannot be reached, 



BIVEB. VALLEYS. 



283 




o K 1 

Plate 25. 
Drainage in the glaciated region of Wisconsin showing the abundant 
swamps (indicated hy dashes) between the drift hills, and the interfer- 
ence of these hills with the stream course. 



I 



284 PHYSICAL GEOGRAPHY. 

because in the conflict between denudation and the internal 
forces of elevation, the latter are more powerful and keep 
the streams either constantly or intermittently at work in 
valley formation. 



REFERENCE BOOKS. 

From the text books of geology, previously referred to, one may obtain 
additional information upon some parts of the subject treated in this chapter. 

Important articles on the Development of Rivers will be found in the 
National Geographic Magazine, Washington, D.C., Volumes I. and II. These 
are from the pen of Professor W. M. Davis. This magazine is a very valu- 
able one for teachers of geography. Six volumes have been published, the 
price to the public being $2.00 each for the first two, and $3.00 for the 
others. To members, they are sold at a lower rate ; and each member received 
the Magazine. The cost of membership is $2.00 a year, and any one inter- 
ested in geography is eligible. 

The remarkable Colorado Canon is fully described by Dutton in Mono- 
graph II (with Atlas), IT. S. Geological Survey, Washington, 1885. $10.00. 
The Atlas is splendidly illustrated. For a shorter account, see Second 
Annual Report IT. S. Geological Survey, 1 Washington, 1882. Powell's 
"Exploration of the Colorado River of the West," Washington, 1872, now 
unfortunately out of print, but still on sale at the second-hand stores, is a 
most fascinating description of travel, as well as a scientific description of 
this wonderful region. The same author has published upon the same 
subject "Canyons of the Colorado." Flood & Vincent, Meadville, Penn., 
1895. 4to. $10.00. 

Huxley. - Physiography. Macmillan & Co., New York, 1891. 12mo. $1.80. 
(A study in physical geography, in which the Thames Basin is taken as 

the central topic.) 

1 Many of the annual reports of this survey may be obtained by the aid of congressmen, 
though the earlier ones are now exhausted. They contain much valuable material, written in a 
sufficiently popular manner for the non-geological reader. Reference is made to many of these 
articles in the later chapters. 



CHAPTER XVI. 

DELTAS, FLOODPLAINS, WATERFALLS, AND LAKES. 

Deltas. — Nearly all streams carry sediment ; and if for any 
reason the velocity is suddenly checked, some of this material 
must be deposited. The most favorable situation for the 
deposit of river sediment, is where the stream enters another 
body of water. In such places the material is deposited 
near the stream mouth, and a delta often results. 

Where streams come from steep mountain valleys upon 
relatively level plateaus, the sudden change in slope causes 
the deposit of some of the sediment at the mountain base. 
This material is dropped most abundantly near the moun- 
tain, and the rapidity of deposit decreases away from it. 
As a result of this, a fan-shaped deposit is produced, to 
which the names alluvial fan, fan delta, or cone delta are 
commonly given (Fig. 155). These deposits are very com- 
mon in arid regions ; and although relatively rare elsewhere, 
when they occur in moist countries, they are usually natter 
and less distinct. The apex of the fan extends up the stream 
toward the mountain base. 

The formation of true lake or ocean deltas, depends upon 
a variety of circumstances. There are many large streams 
which are not forming deltas in the sea. In some cases this 
is due to the fact that the streams carry very little sediment; 
in other cases, the sediment brought to the sea is mostly car- 
ried away by currents. In general, delta formation is not 
favored in open seas, where tidal currents and waves are 

286 



286 



PHYSICAL GEOGRAPHY. 



present to distribute the sediment over the ocean bottom. 
Nearly tideless seas, such as the Gulf of Mexico (Fig. 153), 
or the Mediterranean, are particularly liable to have deltas 
opposite the stream mouths. 




APPROACHES'TO THE MISSISSIPPI RIVER 



Sj>.Armi,JEE 



Fig. 153. 
Delta of the Mississippi. 

If the ocean bottom is sinking, the rate of deposit of mate- 
rial opposite the river mouth is often not sufficiently rapid 
to build a delta above the level of the sea ; and therefore for 
the rapid development of deltas, the ocean bottom must either 
remain in its position, or else be slowly rising. On many 
seacoasts, one or the other of the conditions which favor 



DELTAS, FLOODPLAINS, WATERFALLS, ETC. 287 

delta formation is absent, and this is the reason why deltas 
in the sea are so uncommon. In many cases the submer- 
gence of the coast has transformed the river mouths to 
estuaries, instead of admitting of the formation of deltas. 

By far the most favorable conditions for the formation of 
deltas are found in lakes. Here there are no tides, waves 
are only moderate in effectiveness, and the depth is compara- 
tively shallow and usually not increased by subsidence of the 
bottom. The lake water acts as a filter, removing all the 
sediment which streams bring, and the greater part of this 
is deposited, almost immediately opposite the mouth of the 
tributary. With these very favorable conditions, in nearly 
every lake deltas occur opposite the mouths of most of the 
streams ; and in some cases, by the growth of two deltas 
from opposite sides, lakes are divided into two parts, as at 
Interlaken in Switzerland. 

Over large deltas, the streams flow in uncertain course, 
sometimes changing their channel from one side of the delta 
to the other, as is so frequently done on the delta of the 
Yellow River of China. In this way much destruction of 
life is accomplished. Over the nearly level delta, the main 
stream divides and often subdivides, entering the sea through 
a number of branches, which may be called distributaries, 
in distinction from the tributaries, which bring water to the 
stream, while these distribute the river water to the sea 
(Fig. 153). As a result of this branching of the streams, 
and the changes in river channel, in course of time all parts 
of the delta are traversed by sediment-bringing water ; and 
in this way the delta front is made to advance into the sea, 
while the delta itself is built up above the sea level (Fig. 
154) . In the course of the growth of the delta, the advance 
is often irregular, and arms of the sea may be enclosed in 
the form of lakes (Fig. 153). The form of the delta is 



288 



PHYSICAL GEOGRAPHY. 




roughly triangular, or like the Greek letter Delta (A), 
whence its name. This is really a partial though somewhat 
distorted cone, not unlike the fan delta itself (Fig. 155). 
Floodplains. — Rivers are very often given more load than 

they are able to carry, 
and of necessity they 
are obliged to deposit 
some. The material is 
sometimes deposited in 
the form of bars in the 
stream channel, or at 
other times it is spread over the valley at one side of the 
channel, particularly when the stream has quantities of sedi- 
ment during flood times. In this way, by laying aside parts 
of the sediment load, the stream is forming floodplains. 



ZZZ 



<X4^444X44XX 



Fig. 154. 

Diagram to show the mode of formation of a 

delta. 







Fig. 155. 
An alluvial fan. 



There are numerous ways in which these may be caused. 
They are sometimes merely temporary deposits, being formed 
at the same time that the stream is cutting its channel deeper. 
At certain seasons of the year, the river is obliged for a while, 



DELTAS, FLOODPLAINS, WATERFALLS, ETC. 



289 



and locally, to put aside some of its load, and this it does, 
forming narrow floodplains which are often composed of very 
coarse materials (Fig. 156). We find such floodplains 
very commonly among the mountain streams. 

Usually floodplains are due to a decrease in the river slope, 
a decrease which normally occurs between the headwaters 
and the mouth. Supplied with much material from the 




Fig. 156. 
River bed and floodplain among the mountains. 

upper parts of the valley, the stream reaches these regions of 
less slope with decreased ability to transport the sediment ; 
and some of it must be deposited. This is due to the fact 
that streams are able to transport sediment in proportion to 
their velocity, which itself depends partly upon slope and 
partly upon volume. By far the greater number of the 
large floodplains of the world are due to this decrease in 
river slope, from upper to lower portions. 



290 



PHYSICAL GEOGRAPHY. 



Sometimes the broad flood/plain is in part a delta, which 
has been left inland by the encroachment of the delta upon 
the sea. In the Mississippi valley, the delta began to form 
above the northern limits of the state of Mississippi, and 
has grown outward into the Gulf, filling the estuary which 
existed there, and transforming it to a broad noodplain, as 
we now find it. This change is something like that which 
would happen if the streams now entering Chesapeake Bay 




Fig. 157. 
Floodplain of a great river. 

should fill up the bay, as thev are doing, and change it to a 
level plain composed of fine-grained materials brought down 
by the rivers. 

A change in the level of the land, tilting the seaward por- 
tion of a stream so as to decrease the slope, may also bring 
about conditions favoring the formation of floodplains ; and 
any cause which increases the sediment, also favors this 
formation. If a stream channel is graded to a given volume 
of water and sediment load, an increase in the sediment will 
necessitate the deposit of some, and this will produce a flood- 
plain ; or a decrease in the volume, such as would result in 



DELTAS, FLOOBPLAINS, WATEBFALLS, ETC. 



291 



the change of climate from moist to dry, if the sediment 
load is not also decreased, will bring about floodplain forma- 
tion. From this it is seen that floodplains are formed by 
quite different causes. 

Their characteristics are rather simple. For the most part 
they consist of remarkably level plains (Fig. 157), usually 
partly swampy, and composed of fine soil, which is generally 




Fig. 158. 



so rich that the floodplain regions are important agricul- 
tural districts. The main stream meanders through the 
plain in great swinging curves (Figs. 135, 172, and 157-160), 
so that its course is sometimes greatly increased in length. 
On the Mississippi, a steamer is often within a few hundred 
yards of a portion of the river which can be reached by water 
only by a sail of several miles. These, which are known as 
oxbow curves, are constantly changing in form and hence 



292 



PHYSICAL GEOGRAPHY. 




Fig. 159. 



in position. The river is eating its way into the floodplain 
on the concave bank, and depositing upon the convex bank 

(Figs. 158-160, in which 
the dotted areas repre- 
sent sand deposits). This 
process of change often 
causes the river to cut 
across the narrow neck of 
land between two parts 
of the curve, and thus 
shorten the course and 
abandon the old curve. 
In delta and floodplain 
regions, these are known 
as oxbow cut-offs (Fig. 
159) ; and after they are 
formed, they become crescent-shaped lakes, and sometimes 
they are almost complete circles. In the course of time these 
lakes are destroyed 
by being filled with 
sediment when the 
stream is in flood, 
and when the flood- 
plain is submerged 
beneath the river 
water (Fig. 160). 

These great 
floodplains are con- 
stantly being 
raised by the de- 
posit of sediment ; 
and the time of their formation is that of the flood stage of 
the stream, when it is no longer confined to its channel, but 




Fig. 160. 



DELTAS, FLOODPLAINS, WATEBFALLS, ETC. 293 



overflows and submerges the great level tracts on either side. 
Sediment is being deposited from this great expanse of water, 
because the velocity is decreased in these shallow areas. It 
is to prevent this flooding that the levee banks are built on 
the margin of the floodplain of the Mississippi. These banks 
are built to a sufficient height to shut out the high water 
from the flood- 
plains. 

While the stream 
is constantly at 
work building up 
its floodplain dur- 
ing floods, by its 
meandering it is 
constantly at work 
removing por- 
tions; and so there 
is a process of in- 
termittent move- 
ment of sediment, 
from up stream 
down toward the 
mouth. It is de- 
posited during 
flood ; later it may 
be attacked by the lateral cutting of the stream , and then 
it is carried a step down stream, perhaps to be deposited 
again, and then after awhile to again start in movement. 

Upon a floodplain the tributaries to a river enter the 
main stream at very acute angles. The slope is so gentle, 
that the deposit of sediment near the mouth of the tributary 
constantly tends to divert the river further and further down 
stream. On floodplains, the tributaries often flow for many 





Fig. 161. 
Falls of the Yellowstone. 



294 



PHYSICAL GEOGBAPHT. 



miles in a course nearly parallel to the stream which they 
would join ; and in some rivers, the tributary streams have 
been so far deflected that they enter the sea independently. 

Waterfalls. — When for any reason a stream has a sudden 
descent in its channel, waterfalls or rapids are produced 
(Figs. 161-166) ; and we cannot separate the two phenom- 
ena, because there is every gradation between them. There 
are many ways in which an unnaturally steep slope may be 
introduced into the stream channel. One of the most com- 
mon means is by the accidental diversion of the stream 

from its course. The great 
majority of waterfalls in the 
United States have been 
caused by changes in the 
stream courses, the result 
of some interference on the 
part of glacial deposits. As 
a result of these glacial drift 
accumulations in stream 
valleys, in many cases the 
rivers have been turned to 
one side, and caused to flow 
over steep descents, produc- 
ing either a series of rapids 
or of waterfalls (Fig. 162). 
The thousands of waterfalls 
in northern United States 
are mostly the direct result 
of this kind of accident; and 
Niagara (Figs. 132 and 163) may be taken as a typical illus- 
tration of this kind of waterfall. 

At the close of the glacial epoch, the Niagara River flowed 
from Lake Erie to Ontario along its present course, and 




Fig. 162. 

Taughannock Falls, New York. Caused 
by change in a river course due to 
glacial obstructions. 



DELTAS, FLOODPLAINS, WATERFALLS, ETC. 



295 



entered Ontario after a sudden descent over the bluffs at 
Queenstown (Fig. 169). Glacial deposits left by the ice had 
so filled the old channel, that this new course was the natural 
outflow of Lake Erie. The waterfall produced in this way, 
has been gradually retreating backward toward Lake Erie, 
and at present is seven miles from its former position. In 




Fig. 163. 
American Falls, Niagara. 



the process of this retreat, the gorge has been cut to a depth 
of from 200 to 300 feet, with a width of from 200 to 400 yards, 
while the fall itself is now about 160 feet in height. Careful 
surveys made many years apart, show that the retreat of the 
waterfall toward Lake Erie is rather rapid, on the aver- 
age being not far from five feet a year. If this average 
has been maintained throughout the entire history of Niag- 



296 



PHYSICAL GEOGRAPHY. 



ara, the time occupied in cutting the gorge from Queenstown 
to the base of the falls, is somewhere between 7000 and 
10,000 years. The falls of St. Anthony, in the Mississippi 
valley, are of the same origin, and have had nearly the 
same history ; and the same is true of a vast number of 
waterfalls in the northern states of the Union. 

Any other obstacle in the way of a stream will transform 
it into a waterfall, such for instance as the folding of moun 




Fig. 164. Yosemite Falls. 

tains, or the passage of a lava flow across a stream valley, or 
any one of several similar accidents. When rocks break, 
and move on one side of the crack, as is done when faults 
occur, the movement increases the slope of the stream near 
the fault line. Thus between the plains bordering the 
eastern coast of the United States, and the hilly region just 
inland from these, there is a line of movement, on the land- 
ward side of which the country has been raised; and this 
line has determined the existence of a large number of small 
falls and rapids. Because of this it has been called the fall 



DELTAS, FLOOBPLAINS, WATERFALLS, ETC. 



297 



line; and this small geological accident has been largely- 
responsible for the location of several of the great cities along 
the Atlantic coast. The falls and rapids mark the approxi- 
mate limit of navigable waters, for ships cannot pass over 
them; and since the cities were- so placed in order that they 
might have the advantage of ocean traffic, and still be as far 
inland as possible, they 
were usually located at 
the head of navigation. 
Thus such cities as Phila- 
delphia, Baltimore, Wash- 
ington, and others, are 
situated just on the sea- 
ward side of this fall line, 
and small falls and rapids 
are found almost within 
the city limits. 

As a stream deepens 
its channel, it may actu- 
ally form waterfalls as a 
result of its work. The 
river is able to remove 
soft rocks more rapidly 
than hard ones, and if 
the stream channel is 
crossed by layers of dif- 
ferent hardness, the differ- 
ence in rate of cutting in 
the two kinds of rock will produce a rapid, or even a water- 
fall (Figs. 162, 183, and 165). The hard layer tends to stand 
up above the soft one, and thus there is a steep descent in the 
stream valley. As soon as the stream has cut down to the line 
where its power of deepening ceases, the waterfalls disap- 




Fig. 165. 

Small waterfalls in a gorge near Ithaca, 
N.Y., where the water flows over nearly 
horizontal rocks of varying hardness. 



298 



PHYSICAL GEOGRAPHY. 




pear. Falls of this origin are particularly common in regions 
of horizontal rocks ; for here the waterfall tends to retreat 
upstream (Fig. 166), and hence remains for a long time. In- 
deed, it remains until the stream has eaten its way far enough 
back to have escaped these differences in rock structure. 
There are other causes for waterfalls and rapids, but 
none of especial importance. Perhaps 
one of these kinds should be men- 
tioned •, that is the one so well illus- 
trated in the valley of the Colorado 
River of the West. During times of 
heavy rains, the streams tributary to 
this river bring to the main stream 
vast quantities of material, sometimes 
boulders weighing tons. They are 
able to do this because they enter 
the main stream with very rapid 
slope, — much more rapid than that 
of the Colorado itself. Opposite their 
mouths they build up these coarse 
fragments, which the river itself is 
not able to remove ; and over these bars the water flows 
in rapids, which are sometimes so well developed that 
it is almost impossible to travel down the stream in a boat. 
Only one or two parties have succeeded in passing through 
this canon, and they experienced many dangers which were 
caused by the rapids of this origin. 

Lakes. — A lake is properly a part of a river, and it may 
have been formed by one of several causes. There are many 
differences in lakes ; some are fresh, others salt ; some have 
tributaries from the surface, others are mainly if not entirely 
supplied with water from underground ; some have outlets, 
and others are without them. In form and in depth there is 



Fig. 166. 

To illustrate the probable 
condition at Niagara, 
where the water falls 
over a hard limestone 
stratum, beneath which 
are softer layers. 



DELTAS, FLOODPLAINS, WATERFALLS, ETC. 



299 



almost infinite variety; but in all cases they will be found to 
be parts of river systems. 

Anything that changes a stream valley so that the bottom 
becomes a trough or basin, will produce a lake. By far the 
most common cause for this is the effect of glacial deposits 
(Chapter XVII.). The stream valleys which were carved 
before the ice covered the country, were dammed, or in other 
ways interfered with by glacial deposits, or by glacial action, 
so that when the ice retreated, the rivers found it impos- 
sible to flow over 
the land without 
becoming locally 
transformed to 
lakes (Fig. 167). 
The scores of 
thousands of lakes 
and ponds that 
exist in northern 
United States and 
Europe, are mostly 
due to glacial ac- 
tion (Figs. 168 and 
190). Other acci- 
dents to rivers 
may produce lakes in a similar way. Thus a lava flow 
may dam a stream and form a lake ; or an avalanche may 
do the same ; or the growth of a mountain across a stream 
valley may transform it to a body of quiet water. A large 
majority of the lakes in the world are a result of accidents, 
either of these or other kinds. In many cases the origin is 
complex, several causes uniting to produce the lake basin. 

Not a few lakes in the world are the result of other causes. 
Original depressions on the surface of a land which has 




Pig. 167. 

Avalanche Lake, Adirondack^, N.Y. Part of a river 
valley transformed to a lake. (Copyrighted, 1889, 
by S. R. Stoddard, Glens Falls, N.Y.) 



300 



PHYSICAL GEOGRAPHY. 



been newly added to the continent, when filled with water 
are formed into lakes. This is the origin of the large num- 
ber of lakes in Florida, and of Lake Drummond in the Dis- 
mal Swamp. Others may be produced during and as a 
result of the natural development of streams. Such lakes 
as the oxbow cut-offs described above (Figs. 135 and 160), 
or those formed by the irregular growth of deltas (Fig. 153), 
are dependent upon the development of streams. 




Fig. 168. 
Glacial lakes in the Adirondacks. 



Lakes are merely temporary phenomena, forming but one 
stage in river development. They are speedily removed, and 
any lake which exists in the course of a stream, acts as a bar- 
rier to river development so long as it remains. The removal 
of lakes is usually accomplished by the combination of two 
processes of river work : one, the filling of the lake, the other, 
the cutting of the barrier. Lake-filling is by far the most 
important, and neaily every particle of sediment that come? 
into the lake waters, works towards this end of destruction. 



DELTAS, FLOODPLAINS, WATERFALLS, ETC. 301 

Unless the conditions are exceptional, the process of 
down-cutting at the outlet of the lake is relatively unim- 
portant. For streams which emerge from these quiet bodies 
of water have very little working power, because all sedi- 
ment has been removed by the lake, and the stream has 
thus been robbed of the tools with which it commonly does 
its work. It is still able to act chemically; but this is one of 
the least important means which streams have for cutting 








Fig. 169. 
Bird's-eye view of Niagara gorge and falls. 

their channels. For instance, the Niagara emerges from 
Lake Erie through a valley which is scarcely perceptible 
(Figs. 132 and 169), the river flowing almost on the sur- 
face of the plain; and in all the time that this stream 
has drained Lake Erie, it has done almost no work of 
channel formation between the lake and the falls. 

If the rock which forms the barrier to a lake is composed 
of very soft materials, which the water is easily able to 
remove, or if it is easily soluble, the barrier may be rapidly 



302 



PHYSICAL GEOGBAPHY. 



cut down, and thus the lake be speedily drained. Or if, 
after emerging from the lake, the stream finds itself precipi- 
tated over some steep slope, its power of working is so con- 
centrated by this waterfall, that it rapidly wears a channel, 
as has been done by Niagara between Queenstown and the 
falls. Niagara is wearing back its falls towards Lake Erie ; 
and given time, as a result of this concentration of work, it 
will so lower the outlet as to completely drain Lake Erie. 

Lakes may be partially or entirely destroyed by evapora- 
tion, as has been the case in the great interior basin of the 




Fig. 170. 
Shore lines of extinct Lake Bonneville. 

west. Here there formerly existed numerous large lakes, 
some of which had outlets to the sea (Fig. 151). By a 
change in climate, arid conditions replaced those of moist- 
ness, and the lakes shrunk, until now there exist in their 
place, either alkaline desert plains, or shallow salt lakes 
without outlet. The streams are constantly bringing small 
quantities of salt, and this is gradually accumulated as the 
water evaporates; so that in time, the fresh water becomes 
salt, and this may go on until some of the salt is precipitated 
in the lake bottom. 



DELTAS, FLOOBPLAINS, WATERFALLS, ETC. 



303 



A change to a moist climate would again transform these 
basins to large, fresh- water lakes ; and in the complex 
history of that interior basin region, such an alternation 
of climate has occurred. The geological history reveals 
two moist periods, with intervening dryness; and now 
within sight of Salt Lake City, the beaches (Fig. 170), 
bars and cliffs, formed by the waters of these ancient 
lakes, may be readily seen extending along the mountain 
base. So distinct are they, that even the cowboys have 





Fig. 171. 
A Florida swamp. 



recognized the fact that water formed them. One of these 
extinct lakes, the ancestor of Great Salt Lake (called Lake 
Bonneville), had an area of 19,750 square miles, with a 
depth of 1050 feet. It covered an area now occupied by 
fully 200,000 people, and its depth near the great Mormon 
temple was 850 feet. 

Swamps. — The usual way in which lakes are removed, is 
by the combination of the two processes of filling and down- 
cutting; and generally lake-filling is of more importance 



304 PHYSICAL GEOGRAPHY. 

than the down-cutting of the outlet. In the glacial belt of 
northern United States, where lakes of all sizes were formed 
when the ice retreated, we find abundant illustration of every 
stage in the destruction of lakes. The more shallow of these 
have been transformed to swamps, which are usually a final 

_ ___ stage in the process of 

I lake destruction (Fig. 

rapidity of lake-filling. 

Ray Brook, Adirondacks. . " ° 

At first the plants are 
sedges and other species characteristic of lakes ; then they 
are replaced by mosses ; and finally the swamp becomes 
transformed to a forested area, which is the last step in the 
change from lake to dry land. 

There are other causes for fresh-water swamps. The 
interference with drainage on the part of vegetation, may 
produce swamp conditions. The sphagnum moss, which is 
the form of vegetation causing the peat bogs of the north, 
by growing near the outlet of springs may transform these 
into bog areas, even upon hillsides ; and the growth of 
reeds, and other forms of vegetation, along sluggishly mov- 
ing bodies of water, may transform them into swamp areas. 
The Dismal Swamp, with an area of 1500 square miles, ap- 
pears to be partly due to this cause. The flooding of rivers 
also produces swamp conditions. But by far the largest 
number of swamps are the direct result of the destruction 
of lakes. This is illustrated in the Florida swamps (Fig. 
171), as well as in those of the glacial belt. 



DELTAS, FLOODPLAINS, WATERFALLS, ETC. 305 



REFERENCE BOOKS. 1 

The best treatise upon lakes known to the author is Gilbert's Lake 
Bonneville, Monograph I., U. S. Geological Survey, Washington, 1890. 
$1.50. (A treatise not merely on this one lake, but upon many allied sub- 
jects. An abstract of this appeared in the Second Annual Report, U. S. 
Geological Survey, Washington, 1882.) 

See also Russell's Lake Lahontan, Monograph XL, U. S. Geological 
Survey, Washington, 1885. $1.75. (Short abstract of the same in the' 
Third Annual Report of the Geological Survey, 1883. In later reports 
there are one or two other articles, by the same author, on the ancient lakes 
of the Great Basin.) 

For Swamps, see Shaler, U. S. Geological Survey, Tenth Annual Report, 
Washington, 1890. 

For Niagara, see Gilbert's discussion in the Smithsonian Annual Report 
for 1890 (pages 231-257), Washington, D.C. 

For Shore lines, see references for Chapter XVIII. 

1 The subjects of this chapter, as of some others, are not yet treated in a complete way in 
books of popular interest, and the literature is widely scattered, and often in very inaccessible 
publications. In some of the text books, and general books of reference, these subjects are 
treated from certain standpoints. Some of the monographs of the National Geographic 
Society (published for use in the schools, at the price of $0.20 each) now being issued by the 
American Book Co., New York, promise to fill these gaps. Exact reference cannot be made 
to them, since at the time of writing, only one or two of the preliminarv numbers have been 
issued. 



I 



CHAPTER XVII. 



GLACIERS. 



Cause of Glaciers. — A glacier is an accumulation of snow, 
for the most part solidified into ice, which is engaged in 
a slow movement from one place to another. When the 
snowfall is so great that the warmth of summer is unable 
to entirely re- 
move it, the con- 
ditions favoring 
the formation of 
a glacier are 
brought about. 
Year after year 
the snow accu- 
mulates (Fig- 
173), and in the 
course of time 
this accumula- 
tion makes move- 
ment necessary, 
for it flows ac- 
cording to cer- 
tain laws. As a 
result of this 
movement, particularly when it occurs among mountains, 
the ice stream may extend far below the snow line ; and in 
the Alps, the ends of glaciers are sometimes near fields of 
growing grain. They extend down until they reach a place 

306 




Fig. 173. 
An Alpine snow field. 



GLACIERS. 



307 



where the warmth of the sun is sufficient to melt them, and 
therefore to stop their further movement. This place is not 
a fixed line, but may vary from year to year, so that the 
front of a glacier often retreats and advances. 

The conditions at present favoring the formation of 
glaciers, are found either in high mountains, or else in lati- 




FiG. 174. 
Whitney glacier, Mt. Shasta. 

tudes within the Arctic or Antarctic circles. There was a 
time when these conditions existed further south, and then 
general glaciation was brought about in regions now within 
the temperate zone. There are two quite distinct classes of 
glaciers : the valley or alpine, and the continental glacier. 

Alpine or Valley Glacier This form of glacier receives 

its name from the fact that it is generally developed in 



308 



PHYSICAL GEOGRAPHY. 



mountain valleys, and is particularly well developed among 
the Alps (Fig. 175). We also find valley glaciers among 
most of the mountains of Alaska (Plate 26), in British 
Columbia, in some of the high mountains of Washington, 
such as Mt. Shasta (Fig. 174), and in several places in 
the Sierra Nevadas (Fig. 177). The glaciers of the west 
are small and insignificant, but those of Alaska are among 

the best developed 
in the world. Val- 
ley glaciers are by 
no means uncom- 
mon in other parts 
of the earth ; and, 
among other 
places, we find 
them in Norway, 
New Zealand, and 
Tierra del Fuego. 
In most of the al- 
pine glaciers of 
the northern hem- 
isphere, there is 
evidence that in 
the period imme- 
diately preceding 
the present, they 
extended farther down their valleys than at present. 

The valley glacier has its beginning in the snow field of 
the higher portions of the mountains, which are the great 
feeding grounds (Fig. 67). Here the more level portions 
of the ground are permanently covered with snow, the 
accumulation of many winters. As this increases in depth, 
it is unable to remain on the steeper portions and drops 




Fig. 175. 

The Rhone glacier, showing the ice stream from 
snow field to terminus. 



GLACIERS. 



309 



down the hillsides into the valleys, in the form of great 
snow avalanches. Here it begins a slow movement down 
the valleys, whose slopes are usually steep ; and in the 
course of this movement, the snow becomes compacted 
into ice, and is transformed to the true moving glacier 
(Fig. 175). The rate of movement is exceedingly slow, 
and unless watched very carefully, is not noticeable. In a 
measure, its movement may be compared to that of river water, 
although this comparison is capable of being extended only 
in a very general way. It moves more rapidly in the central 
portion than on the margins, and, like water, it gradually 
moves down the grades. If 
the valley grade is regular, the 
surface of the ice is compara- 
tively smooth, although it may 
here and there be creased by 
fissures or crevasses (Fig. 176). 
When the valley bottom is 
itself very irregular, and the 
slope changeable, the ice top 
may become transformed to a 
very rough surface, which is 
much broken and difficult to 
traverse, and which may be 
called an ice fall. By melting, 
as a result of the effect of the 
sun's rays, the surface of the 
glacier may have its irregu- 
larities increased; and in some 
cases the surface of a valley glacier is almost impassable 
(Fig. 177). 

In the course of its movements down the valley, the 
glacier is engaged in the transportation of a certain amount 




... &S:-I 



\; 



■ 

.--■■■ 



Fig. 171). 
Crevasse in a s. 



310 PHYSICAL GEOGRAPHY. 

of rock material. Some of this is supplied from the valley 
sides, which are subjected to the action of weathering, and 
from which avalanches are not uncommon. As a result of 
this, the margin of the valley glacier is usually lined with 
rock fragments, to which accumulation the name lateral 
moraine is given. Where two valley glaciers unite, the 




Fig. 177. 
Glacier, Mt. Dana, California, showing rough surface and terminal moraine. 

lateral moraines of one side of each glacier join and form a 
moraine in the center, known as the medial moraine (Plate 
26). Some of this rock material escapes through the cracks 
to the bottom of the ice, and this is dragged along the 
bottom, giving to the ice a power which on a large scale is 
not unlike that of sandpaper. The moving ice drags these 
fragments over the bottom, and scours off other fragments 



312 



PHYSICAL GEOGRAPHY. 




Fig. 178. 

Section of a glacier. M, medial ; T, terminal ; and G, 
ground moraines. 



from beneath. This material also is carried by the ice 
down the valley in the form of a ground moraine (Fig. 178). 
After a while, the glacier comes to an end at the place 

where the melt- 
ing is equal to 
the supply of 
ice. Here much 
of the mate- 
rial that was 
brought on the 
back of the ice, or beneath it, is deposited at the frontal 
margin, forming a terminal moraine (Figs. 177 and 178). 
The melting of the glacier furnishes water for a stream, 
which usually emerges 
from an ice cave (Fig. 
179) at the front of the 
glacier, and passes down 
the valley as a muddy 
torrent, carrying with it 
some of the finer parti- 
cles of morainal mate- 
rial. These are the most 
characteristic features of 
the valley glacier. 

A rather peculiar modi- 
fication of valley glaciers 
is found at the base of 
the Mt. St. Elias group 
of Alaska. In these 
mountains, there are 
many large and beauti- 
fully developed valley glaciers (Fig. 67), which, after reaching 
the foot of the mountains, extend toward the sea over a nearlj 




Fig. 179. 
Ice cave at terminus of a glacier. 



GLACIERS. 313 

level plain. The slope of the plain is so slight, and the supply 
of ice so limited, that this part of the united glaciers is almost 
stagnant. There is hardly any perceptible movement; and 
near the margin, morainal material accumulates on the surface 
of the ice in such quantities as to completely bury it, forming 
a soil on its surface, upon which vegetation grOws. We have 
on this, the Malaspina glacier, an instance of a well-developed 
forest, almost as luxuriant 
as some of those found in 
the temperate latitudes, 
but yet growing upon the 
back of a slowly moving 





glacier. A forest also 
extends up to the very 
base of the glacier (Fig. 
180). This form of an , :s , 

ice sheet has been Called Forest at the margin of Malaspina glacier, 

a piedmont glacier, because Alaska. 

it is developed at the foot of mountains. 

Continental Glaciers. — In the arctic and antarctic zones, 
the long winter, and the coolness of the summer, conspire 
to bring about extensive accumulations of snow and ice. 
As a result of this, some of the lands in these cold regions 
are covered with great sheets of ice ; and these are generally 
in movement, from the central portion of the land mass, 
toward the sea. In Greenland, and on the Antarctic land, 
they are so large as to warrant the name continental, for 
they bury lands of continental extent. The Greenland gla- 
cier covers an area of over 500,000 square miles ; and the 
Antarctic ice sheet is several times greater than this. 

From the immense size of the icebergs that float away 
from the margin of the Antarctic ice sheet, we are certain 
that the depth of this glacier is greater than a mile; and 



314 



PHYSICAL GEOGBAPHT. 



there is some reason for thinking that it is nearly two miles 
in depth, even at the margin, while in the interior the 
depth may be over five miles. But about the actual 
conditions existing on this sheet of ice we have very little 
knowledge, for this part of the world is almost entirely 
unexplored. 

Within a few years, our information concerning the Green- 
land ice sheet has become very much increased. Several 
parties have examined it along the coast, and others have 
passed into the interior of the Greenland continent. Near 

the margin, the 
ice extends down 
to the sea, some- 
times as a solid 
wall, but usually 
in the form of 
tongues extend- 
ing down the* 
valleys. The ice 
front is often 
hundreds of feet 
in height, and 
when it extends 
into the ocean, 
bergs are fre- 
quently detached and floated away. Passing from this 
rather irregular margin toward the interior, there is an 
area of rough ice which is difficult to traverse, and through 
which there are some projecting mountain peaks, known to 
the Greenlanders as nunataks (Fig. 181). These rise above 
the great ice field as the only parts of the land exposed to the 
air. Beyond a few miles from the coast, even these high 
mountain peaks disappear, and there is a great ice plateau, 




Fig. 181. 
A nunatak rising above the Greenland ice sheet. 



GLACIERS. 



315 



generally over a mile above the sea, and in some cases hav- 
ing an elevation of about 10,000 feet. 

Whatever the topography of Greenland may be, this 
immense sheet of ice entirely obscures it, and it probably 
covers a land which is mountainous in character. The sur- 
face of the ice in the interior is very smooth, and one may 
travel over it with considerable ease. The movement 
appears to be in all 
directions, from the ^^ 

central part toward *a&\ 

the sea, as if the ^ --'""v*- - '■ :: - ■ 

accumulation were 
greater in the in- 
terior than else- 
where. We can 
form no idea con- 
cerning the depth 
of this sheet of 
ice ; but it is a 
moderate estimate 
to say that it is cer- 
tainly several thou- 
sand feet in depth. 

Icebergs. — The 
cold Arctic winter 
causes the ocean 

surface to become frozen ; and the movement of the waters, 
resulting from the winds, currents, and tides, often breaks 
this ice and throws it into hummocks, so that during this 
season the Arctic water presents a rough ice surface. Dur- 
ing the summer this partly or entirely breaks up, and the ice 
either melts or floats away. Added to this floe ice, are the ice- 
bergs which are derived from the margins of glaciers extend- 





Fig. 182. 
Icebergs in the Antarctic. 



316 



PHYSICAL GEOGRAPHY. 



ing into the ocean (Fig. 182). As the ice moves into the 
sea, the buoyancy of the water causes it to break into frag- 
ments, which then drop into the ocean and drift away. 
Carried by the currents, these bergs may pass hundreds of 
miles from their source; and the Atlantic steamers not un- 
commonly encounter large icebergs that have been derived 
from the Greenland glaciers, while upon the shores of New- 
foundland these are often stranded. An iceberg is mostly 
beneath the water; for, in a regularly formed ice block, 
there are 8.7 parts below the surface of the water for every 
one part that is above. Therefore if an iceberg of regu- 
lar form projects 100 feet 



into the air, there are 870 
feet below the surface of the 
water (Fig. 183). In the 
case of irregular icebergs, 
this may not be true. The 
icebergs from the Greenland 
glacier often extend to a 
height of 100 or 200 feet; 
but those from the Antarctic 
ice sheet are sometimes sev- 




Fig. 183. 

Diagram to show relative proportion of 

submerged ice in an iceberg. 



eral hundred feet above the surface. Some bergs have 
been reported in the Antarctic, which had a height of over 
500 or 600 feet above the water. One such berg ex- 
tended to the height of 580 feet above the sea, and had 
a length of nearly three miles, so that the captain who saw 
it believed it to be an island. Other cases of icebergs with 
a length of over a mile, and a height of more than 500 
feet, have been reported from this region. Such bergs meas- 
ure about a mile from the top to the bottom which is beneath 
the sea. 

Glacial Period: Area covered by Ice. — As was stated in the 



GLACIERS. 317 

last part of Chapter VII., the climatic conditions which we 
now find upon the earth, have not always been the same. 
The most recent and pronounced climatic changes, were 
those which caused the extension of arctic conditions into 
parts of the north temperate zone, and then, later, a change 
from this condition to the present temperate climate. As a 




Fig. 184. 

Glacial lakes and moraine, in a mountain valley not now occupied by a glacier. 

result of these changes, the so-called glacial period was 
caused. This expressed itself in an increase in snow, both 
among the high mountains of the temperate zone, and in the 
higher latitudes. The valley glaciers of Switzerland, the 
Sierra Nevadas, and other mountains, were more extensive 
than at present, and mountain chains in which there are now 
no glaciers, then had their valleys filled with ice streams 



318 PHYSICAL GEOGRAPHY. 

(Fig. 184). But the most remarkable effect, was the pro- 
duction of ice sheets of thoroughly continental character, 
both in northwestern Europe and in northeastern America. 

The entire north temperate zone does not seem to have 
been occupied by a glacier, but there appear to have been 
several large sheets, one set in Europe and another in 
America. It is not certain whether these were connected 
with the Greenland glaciers, but there seems reason to doubt 
whether there was such a connection. The extension of the 



Fig. 185. 
Approximate extension of the continental ice sheet. 

glacier in the United States is shown on the map (Fig. 185). 
The entire region north of the line indicating the terminus 
of the ice, was covered with a glacier which appears to 
have resembled in most respects that which we now find 
on Greenland. Off the New England coast the ice entered 
the ocean, and from it icebergs were discharged ; but in the 
interior, the ice front appears to have changed in position 
from time to time, now advancing, now retreating. Near 
the margin, where the country was mountainous, the higher 
hills projected above the ice in a manner similar to that 



GLACIERS. 319 

noticed along the margin of the Greenland glacier ; but in 
the interior of the ice sheet, the highest mountains appear 
to have been entirely buried. There is evidence that the 
White Mountains of New Hampshire, the Green Mountains 
of Vermont, and the Adirondacks of New York were all 
enveloped in this sheet of ice. 

In Europe the conditions appear to have been similar, and 
the greater part of the British Isles, Scandinavia, Russia, 
and Germany, were covered with an ice sheet, or perhaps 
with several great glaciers moving from different centers. 
Recent studies seem to show that the Greenland ice did not 
have a much greater extension than at present, and that the 
region between America and Europe was not filled with ice. 
So far as we have evidence, there are no signs of extensive 
glaciation in northern Asia ; nor was there on the west coast 
of America, an ice sheet which in point of size would com- 
pare with that of eastern United States and Canada. 

Why the climate changed, cannot be said ; and all that we 
can state definitely is, that we know that there was this 
change. We are not certain how long the ice remained, nor - 
when it came, nor what its detailed history was. We do 
know, that before the glacial period, the climate was not 
frigid ; that the ice occupied the regions for a considerable 
length of time ; and that since then, the conditions have again 
become temperate. Studies of the rate of formation of such 
gorges as those of Niagara, and the Mississippi below the 
falls of St. Anthony, which began when the ice retreated, 
lead to the conclusion that the close of the glacial period 
was probably between 7000 and 10,000 years ago. From the 
geological standpoint, it was one of the most recent chapters 
in the history of the world. 

Terminal Moraine. — The continental ice cap of the glacial 
period behaved very much as the Greenland ice sheet does 



320 PHYSICAL GEOGRAPHY. 

at present. Since no land projects above it, the Greenland 
glacier is not able to carry morainal material upon its sur- 
face ; and the same appears to have been true of the conti- 
nental glaciers of the United States and Europe. Like the 
Greenland glacier, each of these ice sheets moved from some 
central region, in case of eastern America apparently from 
the region of Hudson Bay or Labrador ; and as they moved, 
they dragged rock material from northern towards southern 
regions. When the ice disappeared, much of this material 
was left, just as would be the case if the Greenland glacier 
should melt away. As in the Greenland and valley glaciers, 

the front margin of the ice 
was a place of wastage, at 
which much material was 
accumulated in the form 
of a terminal moraine. 
One of the most distinct 
terminal moraines formed 
by the glacier of the 
United States, follows the 

Boulder in the moraine at Cape Ann, Mass. , ., 

heavily shaded line on the 
map (Fig. 185). Other moraines are found north of this, 
marking stages of halting during the retreat of the ice. 

Both in Europe and America, the glacier has produced a 
very pronounced effect upon the topography and the condi- 
tions of the land surface. There are many details which it 
would be impossible to consider in a work of this kind ; but 
some of the more pronounced features may be mentioned. 
The terminal moraine is one of the most striking topo- 
graphic forms resulting from glacial action. The topography 
is extraordinarily rough and irregular. There are hills and 
hummocks, enclosing valleys and pits, and all are thrown 
together in the most confused manner. The material com- 




GLACIERS. 



321 



posing them is partly clay, partly gravel ; and fragments of 
all sizes, from tiny bits of clay to large boulders (Fig. 186), 
are confusedly thrown together. Sometimes the surface of 




Fig. 187. 

The bear den moraine at Cape Ann, Mass., — a moraine whose surface is covered 

with boulders. 

the moraine is strewn with large boulders (Fig. 187), and 
the morainal material is often 100 or 200 feet in depth, and 

sometimes even more. r _ , 

Formation of Soil. — The 
ice contained much rock 
material derived from more 
northern regions; and when 
it ceased to move, and 
melted away, this was 
dropped at the place which 
it had reached. This 
ground moraine, which is 
commonly known as till 
or boulder clay, forms the soil of the greater part of the 




Fig. 188. 

Boulder-strewn till soil in Maine. Many 
boulders taken from the surface and 
built into walls. 



322 



PHYSICAL GEOGRAPHY. 



-Gimtry included within the glacial limits. It is a clay 
through which boulders of various sizes are scattered 
(Fig. 188) ; and these boulders may often be recognized as 
fragments derived from hills to the north, while the finer 
particles are the result of the grinding action of the moving 
ice. For instance, in central New York many of the bould- 




^1 



A limestone pebble corered with glacial scratches. 



ers have come from the Canadian highlands. The scouring 
action that was in progress beneath the ice, is shown by the 
fact that these boulders and pebbles are finely scratched and 
grooved (Fig. 189); and the same is true of the bed rock 
beneath the soil. At times this till soil is several hundred 
feet in thickness, but usually its depth is only a few feet. 
With the melting of the ice, streams were furnished both 



GLACIERS. 323 

with increased quantities of water, and with increased sup- 
plies of sediment ; and these swollen rivers carried away 
from the ice a large part of the rock material which it bore, 
depositing some in their valleys, and spreading some of it 
over the lowlands. In part, at least, the prairie soil of some 
of the Central States appears to be due to this action of ice 
melting ; and the terraces of many of the streams that 
extended from the ice front, have been derived in the same 
manner. Even a part of the delta of the Mississippi is 
probably built of sediment furnished by the melting ice, 
when the front of the glacier stretched across the head- 
water tributaries of jhis river. 

Formation of Lakes. — Temporary lakes were formed by 
the ice, and in one or two cases these were of great size. 
They were commonly formed where the ice extended across 
streams that flowed toward the north, thus acting as a dam, 
and preventing them from taking their normal courses. 
While hundreds of such lakes were caused, one that 
formed in the valley of the Red River of the North was 
by far the largest and most remarkable of all. This lake, 
which has now disappeared, at one time covered an area of 
110,000 square miles, being 15,000 square miles greater than 
the five Great Lakes combined. It covered the area included 
within the great wheat belt of the Red River valley, in 
Minnesota, North Dakota, and Manitoba ; and Lakes Mani- 
toba, Winnepeg, and Winnipegosis are descendants of this 
great lake, their combined area at present being but 12,500 
square miles. 

Lake Agassiz, as this great temporary water body is called, 
at places had a depth of 500 or 600 feet, and it outflowed 
southward, over the divide in Minnesota, entering the Minne- 
sota River, and passing thence into the Mississippi. Thus 
by this great ice dam, drainage which now finds its escape 



324 



PHYSICAL GEOGRAPHY. 



into the Arctic, was forced to flow in the opposite direction 
and enter the Gulf of Mexico. The proof of the existence 
of this great lake, is found partly in the presence of beaches 
and wave-cut cliffs, now standing high above the bottom of 
the valley, and partly in the great level plain of the Red 
River valley (Fig. 215). The levelness of this plain is due 

to the deposit of sedi~ 
ment in the lake, th& 
bottom being some- 
what like that of Lake* 
Erie. 

Among the othet 
striking effects of the 
glacial period, was the 
formation of many of 
the existing lakes. In 
Minnesota there are 
fully 10,000 lakes and 
ponds which were 
caused by the glacier; 
and throughout the 
Northern States, there 
are scores of thousands 
of glacial lakes (Fig. 
190). Before the ice 
occupied the country, 
the rivers had well- 
established drainage 
lines, and pronounced 
valleys existed. For a time the ice occupied these and 
prevented them from being used as drainage ways. When 
the glacier melted, it deposited the rock materials which 
it was carrying, and deposited these regardless of the 




SCALE OF MILES 



Fig. 190. 
Map of a part of Massachusetts, showing abun 
dance of lakes caused by glacial conditions 
Shaded areas represent lakes. 



GLACIERS. 325 

pre-glacial drainage lines. Sometimes great masses were 
dumped across a stream channel, while in other cases, as for 
instance upon plains, the glacial materials were deposited 
irregularly, so that basins were formed on the drift-covered 
surface. Also, during its movement, the ice appears to 
have deepened some valleys more than others, and some parts 
of valleys more than other portions, thus forming rock basins. 
All of these basins, whatever their cause, were filled with 
standing water when the ice melted, and were thus trans- 
formed to lakes. When the glacier disappeared, the surface 
of the land was dotted with lakes of various sizes and depths, 
and many of them still remain (Figs. 168 and 190), although 
some of the smaller have been destroyed, or transformed to 
swamps (Fig. 172), either by filling or by cutting down the 
gravel barrier. Even the Great Lakes appear to owe their 
origin, in large part, if not entirely, to the action of the ice ; 
and the same is true of the Finger Lakes of central New 
York, of Lake Champlain, and indeed of practically all the 
lakes north of the terminal moraine. 

Formation of Waterfalls. — As a result of the same cause, the 
streams which began to flow after the ice disappeared, were 
often on one side of their pre-glacial channels. Some were 
entirely turned out of their valleys and forced to form new 
ones. Others were only turned aside for short distances ; 
and in some extreme cases, they were actually caused to flow 
over old divides, in an opposite direction from that which 
they had pursued before the beginning of the glacial period. 
The time that has elapsed since the close of the glacial period 
is very brief considered from the geological standpoint ; and 
for this reason, the streams that have been obliged to cut new 
valleys have succeeded in producing only very narrow gorges 
(Frontispiece, and Figs. 133 and 191). The action of cut- 
ting in the channel has exceeded that of weathering, 



326 



PHYSICAL GEOGBAPHY. 



and these young valleys are narrow, steep-sided, canon- 
like gorges, in which waterfalls are common. We find 
illustrations of these post-glacial valleys in almost every 
part of the region occupied by the ice. Side by side we may 
often see the pre-glacial valley, with its broad, gently sloping 
sides, and the narrow, gorge-like channel of post-glacial 

origin. These may often be found 
in the same valley, the stream for 
part of its distance occupying its 
pre-glacial course, and in places be- 
ing in these post-glacial trenches. 

So pronounced has been the effect 
of the ice in the production of lakes 
and waterfalls, that with a fair de- 
gree of accuracy one could map the 
southward extension of the ice sheet 
by merely drawing a line across the 
country, separating the region of 
abundant lakes, waterfalls and 
gorges, from the regions to the 
south, in which these features are 
rare, if not entirely absent. This 
is particularly well illustrated in 
New Jersey where the line runs in 
a westerly direction ; and one can 
see the point well brought out by 
examining a map of a part of Massachusetts, New York, 
Wisconsin, Minnesota, etc., and comparing it with a similar 
map of Kentucky, Virginia, etc. The entire drainage sys- 
tem of the land that was covered by the ice has been rejuve- 
nated, and the details of topography have often been en- 
tirely altered. The great features of hills and valleys are 
practically the same as those which existed before the ice 




Fig. 191. 
A view in Watkins Glen, New 
York, — a post-glacial gorge. 



GLACIERS. 327 

came; but many of the minor details of sculpturing and of 
filling are the result of glacial or post-glacial changes. 



REFERENCE BOOKS. 

Wright. — The Ice Age in North America. Appleton & Co., New York. 

Third edition, 1891. 8vo. $5.00. (From the standpoint of the American 

student, the best book on the subject.) 
Wright. — Man and the Glacial Period. Appleton & Co., New York. 

(International Scientific Series.) 1892. 12mo. $1.75. (Partly based 

on "The Ice Age," being a smaller but very similar book.) 

The subject of British Glacial Geology is treated by Geikie, " The 
Great Ice Age." Stanford, London. (Appleton & Co., New York agents.) 
Third edition. Kevised, 1894. 8vo. $7.50. 

See also Lewis, "The Glacial Geology of Great Britain and Ireland." 
Longmans, Green, & Co., New York, 1894. 8vo. $7.00. 

For Canadian Glaciation, see Dawson, "The Canadian Ice Age." 
Scientific Publishing Co., New York, 1894. 12mo. $2.00. 

Much valuable information and many illustrations are contained in 
Shaler and Davis, " Illustrations of the Earth's Surface, Glaciers." For 
sale by Houghton, Mifflin & Co., Boston, 1881. 4to. $10.00. 

For Moraines, see Chamberlin, Third Annual Report, U. S. Geological 
Survey, 1883. 

For Glacial Striations, see Chamberlin, Seventh Annual Report of the 
same, 1888. 

For Alaskan Glaciers, see Russell, Thirteenth Annual Report of the 
same, 1893. 

For Existing Glaciers of the United States, see Russell, Fifth 
Annual Report of the same, 1885. 

For a statement of Croll's Hypothesis for the cause of the glacial 
period, see his " Climate and Time," referred to at the end of Chapter VII. 



CHAPTER XVIII. 



THE COAST LINE. 



General Statement. — The seacoast is a place of very- 
active change, for here a very slight movement in the land 
registers itself distinctly in the outline of the shore. Mate- 
rials are being brought by various agents and deposited in 
the sea ; and along the shore line, there are ever-acting forces 
which tend to wear back the coast and change the outline. The 
agents of destruction are 

mainly those of waves and • , -j 

associated currents ; and 
the materials removed 
from the coast by wind 
waves, are taken away and 
distributed over the sea 
bottom by wind, tidal and 
ocean currents. There is 
very little difference be- 
tween the coast line fea- 
tures of the sea and those 

of lakes. Waves repeat on the lake shores nearly all the 
features of the ocean shore line (compare Fig. 192 with 
212, and 200 with 213), though usually with less intense 
development. Cliffs and headlands are less pronounced, 
beaches are less extensive, the action of tides is absent, and 
in many minor ways the lake shore lines differ from those 
of the sea; but in general features there is a close resem- 
blance. 

328 




A cliff at Cape Cod, Mass., showing de- 
structive action of waves. 



THE COAST LINE. 329 

Effect of Elevation. — Since the sea bottom is mostly level, 
and since deposits of unconsolidated sediment are spread 
over it (see pages 157 and 164), an elevation of the bottom 
above sea level, usually produces a regular coast line, and 
the materials composing the coast are soft clays or sands. 
There is a general absence of projecting capes, promontories, 
islands, and the smaller irregularities of the coast. The kind 
of shore line that is produced by this cause, is well illustrated 
on the coast of Texas (Fig. 194), although here there have 
been some irregularities introduced by other causes. Great 
sandy beaches, extending for many miles, separate the dry 
land from the sea ; there are no rocks and no high cliffs, but 
sand everywhere. 

Effect of Depression. — The effect of depression of the 
land, or, what would amount to the same thing, the elevation 
of the sea level, produces just the opposite result. Instead 
of causing a regular coast line, it produces marked irregulari- 
ties. If the student could imagine the sea rising to the level 
of the place upon which he lives, he would have some idea 
of the coast irregularities that would result from a depres- 
sion of the land. The sea would rise to the perfectly hori- 
zontal line, and would extend up every valley to the supposed 
level. Some low hills would be entirely submerged, while 
others that rose to heights slightly above the new sea level, 
would form islands. Projecting hills would be transformed 
to promontories or capes, and the stream valleys would either 
become estuaries or bays (Fig. 193). 

In many parts of the world, the last change at present dis- 
tinctly registered along the coast, has been that of submer- 
gence of the land ; and in such places we find an exact repro- 
duction of the conditions imagined. If one examines the 
coast of Maine, as represented upon a good map, it is readily 
seen that the numerous bays and islands are nothing but land- 



330 



PHYSICAL GEOGRAPHY. 



made forms which have been partly submerged beneath 
the sea. Figure 211, representing a part of this coast, 
is a particularly good illustration of these irregularities. 

The coast of northern Eu- 




Fig. 193. 

Coast of Mt. Desert, Maine, showing effect 
of submergence. 



rope illustrates the same 
type; and on the American 
coast, not merely does 
Maine furnish an illustra- 
tion, but from the Arctic 
to Florida, there are abun- 
dant instances of this 
same effect of land move- 
ment. Chesapeake Bay 
(Plate 24) is a land-made 
valley into which the sea 
has entered by submer- 
gence ; and the tributaries to this bay are river valleys also 
partly drowned by the sea. Those who dwell upon these 
coasts find it impossible to say where the river ends and 
the sea begins. 

Thus elevation tends to produce smooth coasts, while de- 
pression introduces irregularities; and since the crust of the 
earth is in almost constant movement, either in one or 
the other of these ways, we find that the general outline of 
the seacoast is usually either very irregular or very smooth. 
In this connection one may well compare the northeastern 
coast of the United States, where the land has recently been 
lowered, with the western coast of South America, where 
the land is rising. 

Effect of Sediment. — The waves and currents in the sea, 
tend to distribute over the sea bottom all mechanical frag- 
ments brought to them from the land, and to form sedimentary 
deposits with them. Generally the sea is able to remove 



V 



THE COAST LINE. 



331 



these materials and to 
deposit them away 
from the coast ; but 
in some cases, the 
amount of sediment 
brought exceeds the 
ability of the oceanic 
agents to remove it. 
This is particularly the 
case at the mouths of 
large rivers where 
deltas are being 
formed. Thus oppo- 
site the mouth of the 
Mississippi, the coast 
is rapidly growing out- 
ward in the form of a 
delta (Fig. 153), and 
the same is true of 
ihe Nile, and many 
other large rivers of 
the world. Even 
where this is not hap- 
pening, the amount of 
sediment brought to 
the sea may so far ex- 
ceed the power of the 
waves to remove it, 
that the coast grows 
outward. Very nearly 
the entire coast, from 
Sandy Hook to the 
northern boundarv of 




Fig. 194. 
Part of an extensive sand bar on the Texas coast. 



332 



PHYSICAL GUOGtiAPllY. 



Florida, is being built outward by the accumulation of 
sediment that the waves have not been able to distribute 
over the sea bottom. This sediment is brought to the sea 
by the rivers, and is piled by the waves into sand banks and 
bars ; and these bars extend as long islands parallel to the 
coast (Fig. 194), being separated from the mainland by 
shallow bodies of water in which salt marshes are often 
present. 

Effect of Waves and Currents. — On exposed coasts, the 




F* 



™l 




Fig. 195. 

View of the island of Heligoland, and map showing how rapidly it is being 
destroyed. Outside line shows boundary in the year 800, when the circumfer- 
ence was 120 miles ; shaded area, boundary in 1300 (circumference, 45 miles) . 
Innermost area, 8 miles in circumference. 



ocean Avaves are constantly beating with such force that 
even the very hardest of rocks are worn away. On the 
European shore, within historic times, this destruction by 
the waves, combined with the action of the tides in remov- 
ing the fragments, has caused the coast to retreat, often for 
distances of several miles. Places that a few hundred years 
ago were at a considerable distance from the coast, are now 
either entirely destroyed, or else are nearer the sea than for- 
merly. On parts of the coast of England, the sea cliffs are 
being worn back at the rate of five or six feet a year ; and 



THE COAST LINE. 



333 





Fig. 196. 
Lake spit. 



it has been estimated that, on a part of the coast of York- 
shire, the shore line has been worn back a distance of two 
miles since the time of the Norman Conquest. Many simi- 
lar cases might be intro- _ 

duced in illustration of this 

wearing back of the coast 

line (Fig. 195). On the 

American coast, we have 

no remarkable instances of 

rapid change ; but still 

there is every reason for 

believing that the shores, 

in certain exposed places, 

are actually being worn 

back at a perceptible rate. 

At the southern end of Martha's Vineyard, the cliffs of Gay 

Head are thus retreating. 

While in some places the action of waves and currents 

is destroying the 
coasts, in others 
it is engaged in 
building them up. 
This was stated in 
the preceding sec- 
tion ; and not only 
is it true in that 
large way, but also 
in a small way. 
The tidal currents 
in the vicinity of 
Nantucket and Martha's Vineyard, on the south side of 
Massachusetts, are moving the sands in such a way that bars 
are being formed, and are almost constantly changing in 




Fig. 197. 
Hook, Lake Michigan. 



334 



PHYSICAL GEOGRAPHY. 



size and position. In some places, where the direction of 
the currents is favorable, permanent bars, or spits, are built 
out from the land (Fig. 196). Sometimes they are curved; 
and such sand bars are known as hooks (Fig. 197). 

According to the conditions under which they are work- 
ing, there is a very marked difference in the action of these 
oceanic agents. On exposed headlands which jut into the 
sea, the action of waves is violent, and the coast line in such 




Fig. 198. 
Sea cave in a well-jointed granite rock, Cape Ann, Mass. 

places is liable to be very precipitous. In enclosed or par- 
tially enclosed estuaries and bays, the wind waves are of 
very little importance, and the changes of the coast line are 
relatively moderate. In harbors, for instance, the wind 
waves are producing almost no change in the coast. Such 
enclosed areas are usually the seat of deposition, instead of 
places in which destructive action is in progress. 

Great difference also results with variations in the kind of 
rock which makes the coast. The waves find it very easy 



THE COAST LINE. 



335 



to cut their way into soft sand and clay, while hard granite 
rocks resist their action. In the hard massive rocks, par- 
ticularly if these are exposed to the action of the oceanic 
waves, there are produced 
cliffs of great size and 
ruggedness. Against the 
base of these, the waves 
dash with violence ; and 
along the line at which 
they are wearing, sea caves 
are* often cut in the rock 
(Fig. 198). The cliff is 
undermined along this line 
of wave action; and, by the 
dropping down of frag- 
ments, it tends to remain in the form of a cliff. Where the 
rocks of the coast are soft, these very precipitous slopes 




Fig. 199. 
Indentation on the coast of Cape Ann, 
Mass., where the waves are removing a 
soft dike rock which crosses the hard 
granite. 




Fig. 200. 
Pond enclosed behind a beach which is built across a small bay, Cape Ann, Mass. 



336 



PHYSICAL GEOGRAPHY. 




L_-.,J 

Fig. 201. 

A crescent-shaped beach, Cape Ann, Mass 



cannot be maintained ; and where a hard rock is crossed by 

a less durable one, the coast is rendered irregular (Fig. 199). 

While these peculiarities of coast line may be found 

developed in many parts of the earth, the tendency of the 

waves and currents is to 
render the coast line al- 
ways more regular. Ma- 
terials are worn by the 
waves from the headlands, 
and drifted into the bays, 
which they tend to fill. 
In the course of time, if 
nothing interferes, this 
material is formed as a bar 
across the mouth of the 
bays, and later is built into a beach, which rises above the 
surface of the water, enclosing the bay as a pond behind 
the beach barrier (Figs. 200 and 213). The material built 
into the beach is usually de- 
posited in the form of seg- 
ments of a circle, concave 
toward the sea, giving the 
well-known crescent-shaped 
beach of the seashore (Fig. 
201). The headlands form 
the two ends of the seg- 
ments, and the material on 
the beach grades from coarse 
pebbles (Fig. 202) near 
the headlands, to fine sand in the central part of the beach. 
The beach is a great mill in which rock fragments are being 
ground by the waves and removed toward the sea (Fig. 203). 
Sometimes these beaches are of great extent ; but almost 




Fig. 202. 

Boulders worn from a headland by ocean 
waves. 



THE COAST LINE. 



337 



always their typical form is tliat of a part of a circle, the 
curve usually being a beautifully swinging curve ; and there 
is a rhythm which appears to bear a relation to wave force 
and direction, and sediment supply. 

Effect of Plants. — It is difficult to estimate the impor- 
tance of the seaweeds which cling to the rocky coasts. 
They form an elastic mat which protects the rock from 
the beating of the waves (Fig. 204); and upon their own 




Fig. 203. 
A rocky beach on the exposed coast of Cape Ann, Mass. 

structure, which is capable of being replaced if damaged, 
they receive the destructive blows of the waves. Along 
the rocky coast of New England, the seaweeds cover the 
rocks from near the line of mid-tide to a depth of several 
feet below the lowest tide, which is the zone where the 
waves are most active. If it were not for this covering, 
these rocky coasts would certainly be worn back with 
much greater rapidity than at present. 

Another way in which plants are active along the shore 



338 



PHYSICAL GEOGRAPHY. 




Fig. 204. 
Mat of seaweed between tides, Cape Ann, Mass. 

line, is in the actual construction of land. On the Florida 
coast there is a peculiar type of tree, the mangrove, which 
has the remarkable habit of growing with its roots in salt 




Fig. 205. 
A mangrove swamp. 



THE COAST LINE. 339 

water. The roots extend into the sea in a network, raising 
the tree trunk above the sea level, as if it were on stilts (Fig. 
205) ; and these root-like branches of the tree encroach upon 
the sea. By this growth of the mangrove, seacoast swamps 
are produced in the shallow waters near the tropics, and 
in this way the coast line is built outward. As the trees die, 




Fig. 206. 
A salt marsh partly rilling an estuary, Cape Ann, Mass. 

their fragments accumulate in the shallow water ; and be- 
tween the roots sediment is entangled, so that little by little 
the land is actually built up and the salt water displaced by 
swamp. 

Even more important than the mangrove, is the action of 
some of the grasses which grow in the shallow water of pro- 



340 PHYSICAL GEOGRAPHY. 

tected bays and estuaries (Fig. 206). These salt marsh and 
eel grasses are able to live where the waves are not too 
violent; and by their growth and death, as well as by their 
action in entangling and causing the deposit of sediment, 
they are important aids in the filling of these shallow 
bays. Along the eastern coast of the United States there 
are thousands of square miles of salt marsh which are in 
large part the result of this action of vegetation. The marsh 
is built up to a level just above that of the highest tide ; and 
along the coast of this region, there is every gradation be- 
tween dry land and the shallow water of enclosed bays, upon 
which the marine vegetation is just beginning to encroach. 
One sees it in almost every bay and estuary, from the Caro- 
linas northward to th« boundary of the country. There are 
vast areas of this salt marsh in the lagoons behind the bars 
which are formed along the southern coast. 

Effect of Animals. — There are many ways in which animals 
are changing the form of the coast line, by far the most im- 
portant being the action of coral animals. These creatures 
are able to live only under certain very favorable conditions. 
The water must be warm, and the temperature must always 
remain above 68°. It must also be clear and free from sedi- 
ment. The animals cannot live in depths greater than 100 
feet, nor can they thrive unless there is a free exposure to 
currents and waves, which bring food to them. Therefore 
we do not find that the coasts of the tropical regions are 
always made by corals. 

Where conditions are favorable, corals thrive in a marvel- 
ous manner (Figs. 79 and 207). They live in an abundance 
that is hardly equaled by any of the other marine animals. 
Each individual builds a skeleton of carbonate of lime, and 
these, combining, form a coral mass, which upon the death of 
the animals, is left behind to enter into the formation of a 



THE COAST LINE. 



341 



limestone rock. The corals grow along the coast, forming 
large reefs ; and at times they produce reefs at a considerable 
distance from the shore, which are then known as barrier 
reefs. The Great 
Barrier Reef of ^ 
Australia ex- 
tends along the 
coast, with some 
interruption, for 
a distance of 
1000 miles ; and 
at times its dis- 
tance is 60 miles 
from the shore, 
being the most 
extensive growth 
of coral in any 
single region in 
the world. Its 

width at the surface is rarely more than one or two miles. 

At times the coral builds isolated islets, which are often 

known as keys, and which are so well illustrated by the keys 





Fig. 207. 
A coral reef on the Australian coast. 



SCALE OF MILES 


.•/ ..,-; 


12 3 4 5 

/MARQUESAS^ 


£ f COTTRELL KEY -... "8* '-.. 

•£■ ~" MULE KEY= Q MULLET KEY .•'.". %. 


(?' KEYS ]g 


*. / "C )i_ SNIPE KEY OR A^j* *V jy\ 

§ 1 KEYCP>- KEVB /"" 
^ BOCA GRANDER WOMAN KEY»- ,■■' 

MAN KEY 1 ' 


S.D.Strm... 2T.T. 


TTest Ch*« ntX 




Fig. 208. 


Atoll-like 


keys on the Florida coast. 



342 



PHYSICAL GEOGRAPHY. 



at the southern end of Florida (Fig. 208). In the mid- 
ocean, particularly in the South Pacific, the coral growth 
forms ring -like islands, which are known as atolls (Fig. 209). 
Sometimes these are nearly perfect rings, enclosing an area 
of water which is connected with the sea by a small opening. 
The atoll rises above the level of the sea to a height suffi- 
cient for the growth of trees, and many of these islands are 
inhabited by man. The reason for their elevation above the 
sea is the washing action of the waves, combined with the 




Fig. 209. 
An atoll in the South Pacific. 



blowing of the wind, which drifts the coral sand into mounds. 
On all of these reefs, corals are still living and growing where 
exposed to the action of the waves and currents which are 
bringing food to them. It is found that the coral reefs are 
better developed on coasts which are exposed to the oceanic 
currents of tropical origin. As is so well illustrated in the 
Bermuda Islands, which are in latitude 32° N., corals may be 
developed where these currents extend their warmth into 
latitudes well beyond the tropics. ■ 



THE COAST LINE. 



343 




The cause for atolls is at present in dispute, and it does 
not seem desirable to consider the question as to which ex- 
planation is correct. The one which has been before us for 
the longest time (having been proposed by Darwin), and is 
accepted by many geologists, is that the atolls are nothing 
more than reefs which once surrounded volcanoes that have 
since disappeared by submergence (Fig. 210). As the cone 
sank beneath the 
water, the corals 
built the reef higher 
and higher, so that 
even after the cone 
had entirely disap- 
peared, its position 
was indicated by the 
ring-like reef. Cer- 
tainly this seems to 
be a true explanation 
for some atoll reefs ; but for others, another explanation is 
very likely necessary. The great barrier and fringing reefs 
are merely formed by the growth of the coral along or near 
the coast line. 

Changes in Coast Form. — With change in the conditions, 
a coast may assume entirely different characteristics. If, for 
instance, the clear waters of some coral coasts are for any 
reason changed to muddy water, coral life is driven out, and 
the muddy or sandy shore takes its place. If land is ele- 
vated, or if it is depressed, the form of the coast is very 
greatly changed. The agents of denudation are always at 
work tending to alter the coast form. Therefore the shore 
line which we know at present, is merely a temporary fea- 
ture, merely the stage which has been reached at the present 
time; and it is far from the condition which has existed in 



Fig. 210. 

Diagram to illustrate one explanation of the 
origin of atolls. V, volcanic cone; aa, bb, 
cc, successive levels of the sea ; del, ee, and ii 
showing corresponding condition of the reef, 
finally producing the atoll ii when the volcano 
was entirely submerged. 



344 PHYSICAL GEOGRAPHY. 

the past, and probably from the condition which will exist 
in the future. In imagination we are able to look back to 
the time when the eastern coast of the United States had 
not its present irregularity; and by geological evidence we 
are also certain that but a short time ago Florida was 
not present as a peninsula. The delta of the Mississippi is 
a growth of very recent date ; and preceding its formation 
an estuary extended up the valley of the Mississippi, at least 
as far as Arkansas. 

Our knowledge of the geology of the coast line is not suf- 
ficiently detailed to allow us to study all the changes that 
are going on ; but any one who dwells by the coast, will be 
able to see that there are some changes now in progress. A 
visit to the seacoast in time of storm, or indeed to the lake 
shore, will convince any one that there are changes in 
progress, which, as a result of the repetition of this action 
through scores of years, must produce perceptible changes. 

Islands. — There is a very great variation in the size of 
oceanic islands, in the distance from the shore, in the form, 
and in origin. It is quite customary to speak of two classes 
of islands : oceanic and continental, the oceanic being those 
which occur far from the land. These oceanic islands are 
generally of three classes : (1) those that are formed by 
volcanoes, (2) those that are produced by the folding of 
mountains, and (3) the mid-ocean coral reefs. Generally 
they are small, and they often occur in chains, as if they 
represented tops of mountain peaks along some ridge that is 
partly beneath the ocean. In some instances, soundings have 
shown that this is actually the case. 

Near the coasts of continents we have the same kinds of 
islands. The Japanese archipelago is apparently a moun- 
tain chain which is now in process of being formed. In the 
Mediterranean, among the West Indies, and elsewhere, there 



THE COAST LINE. 



345 



are many instances of volcanic peaks which form islands not 
far from the coast ; but probably nine out of ten of the 
islands of the world have resulted from causes other than 
these. Most of the islands are derived either from the 
submergence of the land (Fig. 211), or from the building up 




m. 



/^fj Same's Pt 
Moore's neJk' Pt. ^\V^ 

^'LANE'S I 

Parker's 



r/Wm 

, fly<lng?pt. MareJ 



JTTi UPPER^A 

C^ GOOSEfl./ 

BER'S I. S\\So 



ji MM'*!* 



l&OSE I. 'd" 







M;f ^ 



Potts' t:: j /^~ Q s?' {j 
TTLE BANGS,!./ Harbor)/-' ^ JfL 

K HOPE^V 



SjPr-Jy IS ) HORSE l.f^Y 








5 E.D.S, r ™,i.N.Y. 



Fig. 211. 
The islands, capes, and promontories of Casco Bay, Me. 

of the coast line by wave action (Fig. 191). As has been 
stated, the waves throw up bars in favorable places, the 
wind gathers the sand and blows it into sand dunes, and 
islands of this origin are found in abundance along many 
coasts. South of Cape Hatteras there is a line of such 
islands, which are due partly to wave and partly to wind 



346 PHYSICAL GEOGRAPHY. 

action, and which stretch along the coast parallel to the 
mainland, and but a short distance from it. 

When an irregular coast is lowered, the sea rises around 
the hills, forming islands. This is excellently illustrated 
on the coast of Maine (Fig. 211), where the thousands of 
islands and islets are in most instances the direct result of 
the partial lowering of hilly land beneath the sea level. 
There are some minor ways in which islands are produced, 
but these are by far the most important. 

Being surrounded entirely by water, islands are peculiarly 
liable to destruction by wave action (Fig. 195). They are 
open to attack from every side, and if they happen to be in 
the ocean far from the land, they may be very rapidly de- 
stroyed. Even extensive volcanoes in the mid-ocean are 
quickly worn away as soon as the volcanic fires have ceased 
to add material in place of that which the sea removes. 
Among the Hawaiian Islands, the smaller islands, which were 
formed by volcanoes now extinct, are rapidly being destroyed; 
and the same is noticed throughout the Pacific, as well as 
among the volcanic islands of the Atlantic (Fig. 125). A 
volcano formed in the Mediterranean, in 1831, and known as 
Graham's Island, reached a height of 200 feet, with a circum- 
ference of three miles ; but in the course of a few years it 
was entirely destroyed by wave action. 

Promontories. — Capes and promontories belong to the 
same class of seashore forms, the difference being merely in 
size. They are produced in several ways. Some of the 
largest promontories are parts of mountain folds in the sea. 
Others are areas of hard rock which have resisted the agents 
of denudation and formed highlands, which, when a sub- 
mergence of the land took place, remained above sea level, 
while the lower, neighboring parts of the land were sub- 
merged. Labrador and Nova Scotia illustrate this. 



THE COAST LINE. 



34? 



By far the greater number of capes are a result of this 
kind of submergence of land (Fig. 211). The peninsula 
of Florida owes its existence, in part at least, to the action 
of corals, which have built the southern half. The warm 
Gulf Stream, which bathes this coast, has brought food to 
the coral animals, and these have built reefs. In the vicinity 
of Key West the peninsula of Florida is still growing south 




Fig. 212. 
A bluff cut in clay, on the Lake Michigan shore. 

ward, the keys being merely small parts of a submarine 
plateau which are above the sea. Some small capes are 
caused by the building out of the land through the deposit 
of sediment. Sandy Hook is an illustration of this, and 
many of the hooks and spits of sandy coasts are of the same 
origin (Figs. 196 and 197). 

Lake Shores. — On the shores of lakes we have instances 
of changes due to wave action, of cliffs formed by the under- 



348 



PHYSICAL GEOGRAPHY. 



cutting of waves (Fig. 212), of the building of bars and 
hooks (Fig. 197), of the formation of beaches (Fig. 213), 
and indeed of nearly all the phenomena of the seashore. 
Since many lakes are nothing but river valleys that have 
been dammed through some agency, both islands and capes 
are often produced. These occur where the necessary 
irregularities existed on the side of the valley which has 

been filled with 
water. Where 
there were trib- 
utaries to the 
stream which has 
been transform- 
ed to a lake, the 
lake water enters 
in the form of a 
bay ; and the 
hillside border- 
ing this bay ex- 
tends into the 
lake as a cape. Sometimes there are hundreds of islands 
in the lake waters. The Thousand Islands, at the outlet of 
Ontario, are the tops of low hills, the sides of which are 
submerged beneath the lake waters. 

In the lakes there is usually less violent action than on the 
seashore, partly because the waves do not rise to the height 
of the true ocean wave, and partly because the tide action is 
absent. The shores of most of the smaller lakes bear a closer 
resemblance to those of the partly enclosed harbors and bays 
of the seacoast, than they do to the exposed ocean coasts. 
But except in intensity of development, there is little differ- 
ence between lake shores and seacoasts. 

Fossil Shore Lines. — Coast lines are sometimes abandoned, 




Fig. 213. 

A lagoon enclosed behind a beach barrier on the shore 
of Lake Michigan. 



THE COAST LINE. 349 

and may then be found on the dry land. This happens 
when the land on the seashore rises, or when for any 
reason a lake disappears. These shore lines have all the 
features of those now forming in the sea or lake. There are 
fossil beaches (Fig. 170), bars, spits, hooks, wave-cut cliffs, 
etc. ; and immediately after their abandonment by the waves 
their features are very distinct ; but in a short time they 
begin to crumble away under the action of denudation, and 
before long no sign is left to tell of the change. Such 
ancient shore lines on the coast of New England and 
Labrador, tell of a recent submergence of the land ; and 
the shore lines south of the Great Lakes, tell us that they 
once covered a considerable area of country south of their 
present site. 



REFERENCE BOOKS. 

Much of interest is found in Gilbert's "Lake Bonneville," referred to at the 
end of Chapter XVI. A special paper on shore lines, by the same author, is 
found in the Fifth Annual Report U. S. Geological Survey, Washington, 
1885. 
Shaler. — Sea and Land. Scribner, New York, 1894. 8vo. $2.50. (This 

contains much of value upon shore lines, written in Professor Shaler' s 

remarkably entertaining style.) 

Dana. — Corals and Coral Islands. Dodd, Mead & Co., New York, 1890. 
8vo. $5.00. 

Darwin. — The Structure and Distribution of Coral Reefs. Smith, 
Elder & Co., London (Appleton & Co., New York, Agents). Third Edi- 
tion, 1889. 12mo. $2.00. 
Eor Harbors, see Shaler, Thirteenth Annual Report U. S. Geological 

Survey, Washington, 1893. 

For Salt Marshes, see Shaler, Sixth Annual Report of the same, 1885. 



CHAPTER XIX. 



PLATEAUS AND MOUNTAINS. 



r 



Fig. 214. 
Pecos River valley, southern New Mexico. 



Plateaus. — A plateau is a level-topped area at a consider- 
able elevation above the sea. In many respects it resembles 

a plain (Figs. 214 and 
215), but usually it is not 
so level; and the ordi- 
nary distinction between 
plains and plateaus is 
based upon elevation. 
Both are relatively level 
areas ; and both plains 
and plateaus are usually 
composed of sedimentary rocks in a nearly horizontal position. 

Plateaus are gen- 
erally associated with 
mountains, and most 
mountains rise above 
a basal platform 
which is a true pla- 
teau. Thus at the 
eastern base of the 
Rocky Mountains, 
the plateau of the 
Mississippi valley Fig. 215. 

ascends to the verv Plain i n tne valley of the Red River of the North. 

mountain base, while on the western side there is the ex- 
tensive interior plateau of the Great Basin. In nearly 

350 




PLATEAUS AND MOUNTAINS. 



351 



every continent there are large plateau areas, but nowhere 
is this form of topography better developed than in the 
central part of Asia, north of the Himalayas, where for 
thousands of square miles the plateau rises to an eleva- 
tion of several thousand feet. A portion of the Indian 
plateau region, as well as a part of the plateau of the Rocky 
Mountain area, is covered with an extensive series of lava 
flows which have been sent to the surface through great fis- 
sures. The lava-capped plateau of the Deccan has an area 

of 200,000 square miles, r _ ...... 

and that of the Snake 
River valley of Idaho also 
covers an immense area, 
with a depth in places 
greater than 3000 feet. 

Since they are in associa- 
tion with mountains, these 
plateaus are very liable 
to be arid. Many of the 
interior deserts between 
mountain ranges are real 
plateaus, as is so Avell illus- 
trated in the western part of our own country. Among the 
various mountains of the western half of the continent, the 
prevailing condition is that of arid plateaus broken by occa- 
sional mountain ranges ; and the conditions of dryness and 
absence of forest-covering, exist also on the plateau east of 
the Rocky Mountains. This plateau region is usually spoken 
of as the Plains of the Far West. In the general levelness 
of the surface, and also in the absence of forest, it resembles 
the prairies east of the Mississippi. But in the case of the 
prairie, the forest is not absent because of dryness, while this 
is the cause for its absence in the so-called Plains. 




Fig. 216. 

Taos Mountains, New Mexico, rising above 
an extensive plateau. 



352 PHYSICAL GEOGRAPHY. 

Since plateaus are elevated above the general level of the 
country, they are often very deeply carved by river erosion. 
Some of the most remarkable cases of deep, narrow river 
valleys are found among high plateaus. Nowhere is this 
better illustrated than in the high plateau of Utah and 
Arizona, through which is cut the remarkable canon of the 
Colorado (Fig. 142 and Plate 28). In this respect also, 
there is a difference between plateaus and low plains ; for 




f /. 




b • 


_ ., . 




Fig. 217. 




The plateau near the Colorado River. 



the latter are not crossed by deep valleys, because their 
surface is usually not far above the level of the sea. 

As is so strikingly shown in the canon of the Colorado, the 
ruggedness of the river valley formed among plateaus, de- 
pends in no small degree upon the climatic conditions. The 
climate is so dry that the agents of weathering are not very 
important ; and consequently the main work of sculpturing is 
done by the river itself. This causes a deep, angular trench, 
whose angularity is preserved because the rocks which border 



PLATEAUS AND MOUNTAINS. 



353 



the valley are not rapidly melted away under the action of 
rain and frost. As a result of this peculiarity, the charac- 
teristic topography of the plateau in an arid region, is that 
of occasional level stretches with steeply sloping boundaries 
(Figs. 142 and 217). The country is often cut into a series 
of terraces, one step above and beyond another. In the 
western part of this country, the level-topped sections of the 
plateau have been given the name of mesa, which means 
table ; and when the level-topped sections are small, they are 
called buttes (Figs. 218 and 257). 

Mountains : Characteris- 
tics of Mountains. — Popu- 
larly considered, a moun- 
tain is any unusual 
elevation ; and upon the 
plains of Texas, an eleva- 
tion of 100 or 200 feet 
passes as a mountain, while 
in a thoroughly mountain- 
ous district, elevations of 
1000 or 2000 feet are known 
as hills. We shall accept 
this common usage of the 
term mountains ; but it will be pointed out that there are 
various kinds, derived in a variety of ways. By far the 
greater number of mountains, and certainly the most pro- 
nounced in the world, are the direct result of folding of 
the earth's crust, in nearly all cases combined with a great 
amount of destructive action of denudation. Along certain 
lines, the rocks of the crust are folded and broken into great 
ridges and chains, which in some cases extend from one end 
of a continent to another. Indeed, in the American conti- 
nents there is practically one continuous set of rock folds, 
2a 




Fig. 218. 
Butte in New Mexico. 



354 PHYSICAL GEOGRAPHY. 

from the southern end of South America to the northern 
part of Alaska. 

A set of rock folds forming a great mountain series is 
generally known as a system. The Rockies form a system 
of mountains, and several systems combined form a cordillera 
(Fig. 129). This is illustrated in the western part of this 
country, which is crossed not only by the Rocky Mountains, 




Fig. 219. 
A talus slope at the base of a mountain ridge. (Elk Mountains, Colorado.) 

but by the Basin Ranges, the Sierra Nevadas, and the Coast 
Ranges. When we examine any single mountain system, 
as, for instance, that of the Rockies, we find it to be com- 
posed of various parts. There are individual ranges among 
these mountains, and any single range is also found to be 
composed of separate parts, to which the name ridges (Fig. 
219) may be given. In all of these cases, the striking pecui- 



PLATEAUS AND MOUNTAINS. 



355 



iarity is that the length of the mountains greatly exceeds 
both the width and the height. The cordillera and the 
system may extend for a thousand or more miles ; the 
range may extend for a distance of more than a hundred 
miles ; hut the ridge is usually only a few miles, or at most 
a few score of miles, in extent. 

There are prominent parts of mountains which do not 
have this charac- 
teristic of the F 
ridge, and these 
are spoken of as 
peaks (Fig. 220). 
In nearly all cases j 
the real mountain 
peak is merely a 
portion of a ridge 
or chain, which 
for some reason 
stands up higher 
than the sur- 
rounding parts. 
The usual cause 
for this greater 
elevation of one 
portion, is the 
presence of some hard rock which resists weathering. While 
mountains are forming, and after they have been formed, 
they are subjected to the agents of denudation, which tend 
to wear them away; and in this process of destruction, the 
harder rocks are left higher than the softer ones. In look- 
ing among the more pronounced mountain peaks of the 
world, we find that in most cases these are made of some 
particularly durable rock. Pike's Peak is made of granite ; 




Fig. 220. 
Matterhorn, a Swiss mountain peak. 



356 PHYSICAL GEOGRAPHY. 

the Matterhorn (Fig. 220) of the Alps is composed of a 
similar hard crystalline rock, and the White Mountains of 
New Hampshire, the peaks of the Adirondacks, etc., have 
the same characteristic. This is the typical mountain peak, 
a form resulting partly from the folding of the rocks during 
the formation of the mountains, partly from the differences 
in the hardness of rock, and partly from denudation. In 
the longitudinal parts of mountains, the fold is the most 
prominent factor ; in these more nearly circular portions, 
the factor of prominence is rather that of denudation. 

There are other forms of mountain peaks in which rock 
folding does not enter as a prominent cause. The most 
abundant of these are the volcanic peaks whose origin and 
characteristics are discussed in the next chapter. In many 
parts of the world, particularly on plateaus, there is a form 
of elevation often called a mountain, which is the result 
merely of denudation acting upon strata whose position is 
nearly horizontal. There has been no folding, and no dis- 
turbance of the rocks other than that of elevation ; but hills 
or peaks have been cut out by erosion, and these now stand 
above the general level of the country. In the western 
part of the United States they are often knoAvn as buttes. 
By some they are called hills of circumdenudation, because 
all around the elevated portion the rocks have been cut away 
(Figs. 218 and 257). 

Next in prominence to the elevations of the mountains are 
the depressions. Between the ridges, systems, and peaks, 
there are valleys ; and these have quite distinct character- 
istics. Between systems, and really forming a natural part 
of cordilleras, there are often great valleys, sometimes hun- 
dreds of miles in width and length, to which the name inte- 
rior basin is generally given. They are great plateau areas 
between mountain walls, and they are usually more or less 



PLATEAUS AND MOUNTAINS. 



357 



broken by mountain ridges. Sometimes, in part of their 
area, there is drainage to the sea ; but very often, and as 
a characteristic feature, a part of the drainage finds its way 
into these great troughs, and does not escape to the sea, but 
is returned to the air by evaporation. 

The Great Basin of the United States has an area of over 
200,000 square miles; but notwithstanding the great size of 




Fig. 221. 
A mountain park (Baker's). 

the basins of interior drainage on this continent, these form 
but 3.2 percent of the total continental area. In Australia 
nearly 52 per cent of the area is in the condition of interior 
drainage, while 31 per cent of Africa is in the same con« 
dition, and 28 per cent of the continental mass of Eurasia 
is an enclosed basin. The Sahara interior basin is 16 times 
as large as our Great Basin, and the interior basin region of 
Asia occupies an area 23 times as great as that of the west. 



358 



PHYSICAL GEOGRAPHY. 



Between mountain ridges and chains, there are often 
longitudinal valleys of considerable size, extending par- 
allel to the chains between which they occur. These are 
among the striking features of mountains, and they are 
generally occupied by streams which are evidently too small 
to have carved such immense valleys. When the rock 
structure is studied, it is evident that these valleys repre- 
sent either down-folded portions of the crust, or else portions 




Fig. 222. 
A mountain gorge in the high Andes of Peru. 

that have been broken or faulted down. Where these val- 
leys occur between peaks and ridges, forming amphitheaters 
among the mountains, they produce a characteristic valley, 
which among the Rocky Mountains is given the name of 
park (Fig. 221). 

Occasionally the streams have carved mountain gorges, 
and even in some cases have cut entirely across the ridges, 
forming valleys which are characterized by remarkably steep- 
sided gorges (Fig. 144). They furnish some of the most 



PLATEAUS AND MOUNTAINS. 



asy 



striking bits of mountain scenery, and in traveling across a 
mountain riclge upon a railroad, one is often carried through 
these gorges, which furnish the sole means of easy passage 
for the railroad (Figs. 134 and 222). Low points in moun- 
tain ridges are known as passes. Sometimes these are 
merely parts of the mountain which were not folded so 
high as other portions; but in many cases they are valleys 




Fig. 223. 

Mount of the Holy Cross, Colorado — above the timber line. 

at the headwaters of streams. Two streams head together 
in a mountain ridge, and these lower the ridge at this 
point, producing a gap, which is usually taken advantage 
of as a means of passage across the mountains. 

Mountains in their best development are extraordinarily 
rugged. They rise in a series of slopes, sometimes moder- 
ate, but at other times very precipitous. They are cut by 



360 



PHYSICAL GEOGRAPHY. 



valleys which are often bounded by true precipices. The 
hard rocks stand up precipitously, while the softer strata 
furnish more gentle slopes. The mountain form, in all of 
its irregularity and variety, depends upon the action of the 
agents of denudation upon the rocks of different hardness 
which have been folded into more or less complex attitudes. 
Generally the mountains are regions of heavy rainfall ; but 
if they rise to a very considerable elevation, this comes 
mostly in the form of snow ; and even within the tropics, 




Fig. 224. 
Trail on Long's Peak, Colorado. 



the high mountain peaks may be snow-capped throughout 
the year. Near the base of the mountains, the fact of heavy 
rainfall causes the growth of luxuriant vegetation, generally 
in the form of a dense forest covering. As one ascends the 
mountain sides toward the upper regions of cold, the forest 
gradually changes in character, at first assuming the habit 
of the northern forest, then becoming more and more sparse 
(Fig. 221), and finally, when the timber line is reached, 
entirely disappearing (Figs. Q6 and 223). At the timber 



PLATEAUS AND MOUNTAINS. 



361 



line the forest is replaced by scattered patches of trees ; and 
above this, these forms of vegetation disappear. 

As these upper regions of the mountains are approached, 
the peak becomes more and more rugged. Generally the 
surface of the ground is strewn with loose boulders, which 
have been broken from the rock that formed the peak 




Fig. 225. 
Mountain ridge on the Canadian Pacific. 

(Fig. 224). They have been removed from the ledge by 
the action of frost, and are being disintegrated. Upon these 
mountain peaks, because of the great cold, frost action is 
very important. By removing all loose particles, the violent 
winds check the formation of soil, and the excessive slopes 
also tend to prevent this ; for every drop of water that falls, 
passes down the steep incline, carrying along all small frag- 



362 PHYSICAL GEOGRAPHY. 

ments. The absence of plants removes a protective covering 
that is important in modifying the action of weathering. 

The form and ruggedness of the mountain chain, ridge, 
or peak will depend upon a variety of circumstances, chief 
among which are the kind of action which has formed the 
mountain, the position and structure of the rocks out of 
which the mountain is made, and the length of time during 
which denudation has been acting toward the destruction of 
the mountains. Where there are unusually hard layers in 
a mountain ridge, these tend to remain high above the sur- 
rounding country, and the mountain always has the ridge- 
like form (Fig. 225) ; but where the ridge itself has been 
subjected to variations in folding, in the course of time its 
ridge-like habit may be destroyed. The massiveness of the 
rocks forming the mountains also has much to do with their 
ruggedness. If composed of a series of strata of irregular 
hardness (Figs. 261 and 262), the topography will be very 
different from that resulting in a mountain composed of 
rocks of uniform character (Fig. 251). The most precipi- 
tous and rugged of mountains are those made out of rocks 
of uniform structure. Some of the ridges in the Rockies 
are made of massive limestone, and among these there are 
excessively high precipices. 

The Origin of Mountains. — Several theories have been pro- 
posed to account for the formation of mountain folds; but 
at the present time no one of these can be said to be thor- 
oughly satisfactory. We are in doubt as to the actual 
reason for the folding of the surface rocks along certain 
lines. This much is quite universally agreed upon, — that, in 
one way or another, it is the result of the heated condition 
of the interior of the earth. The greater number of geolo- 
gists also believe that the most satisfactory explanation at 
present before us, is the one depending upon contraction. 



PLATEAUS AND MOUNTAINS. 363 

The interior is highly heated, and this heat is passing from 
the earth into space. As it is lost, the heated interior also 
necessarily loses bulk, and the cold solid crust attempts to 
accommodate itself to this constantly decreasing interior. 
The crust itself does not lose in bulk, and in order to sur- 
round the sphere, which is constantly having its diameter 
shortened, it must wrinkle ; and the comparison is very well 
made between this supposed action of the crust of the earth, 
and that which happens when an apple is dried by exposure 
to the air. As the apple dries, water passes from within, and 
the interior portion constantly loses in size, while the skin 
does not lose bulk, but always attempts to surround the 
apple, and in doing so produces a wrinkled surface. 

The mountain and continent folds, and indeed all of the 
expressions of frequent movement of the earth's crust, are 
believed by many geologists to be the direct result of this 
contraction of the interior ; and this theory for the forma- 
tion of mountains is known as the contraction theory. It is 
possible that there are other causes aiding, and it cannot be 
denied that there is a possibility of some other explanation. 
Our knowledge of the interior of the earth is too limited 
to warrant any dogmatic assertion upon hypothesis. 

The growth of mountains is not a stupendous overturning 
along certain lines, but rather a very slow upward or down- 
ward folding of a portion of the rocks. From all the evi- 
dence that we possess, there is no reason for believing that 
any mountain chain in the world has ever grown with sud- 
denness. There is reason for believing that the Coast 
Ranges of the Pacific slope are even now in the process 
of growth, and this is certainly true of the Japanese Islands 
and of the Andes. So far as we may judge, these two latter 
instances are illustrations of rather rapid mountain growth; 
I and yet, in both places, people find it possible to live with no 



364 



PHYSICAL GEOGRAPHY. 



other danger than that coming from occasional volcanic erup- 
tions and earthquake shocks. The crust of the earth is 
not convulsed, but is folded with slowness. This is true 
even when the rocks break instead of bending. Faults, 
representing the breaking of the rocks along certain planes, 
are even now in process of formation in various parts of the 
world. 

If we examine a section of a mountain, we find the rock 
strata extending from the earth on either side of the ridge 
(Fig. 226); but their extension into the air has been pre- 
vented by de- 

„-''', "\ nudation. The 

edges of the 
rock layers 
have been trun- 
cated by this 
action. If we 
continue the 
strata from one 

side to the other, joining like layers (Fig. 226), we find 
that a mountain would result whose height would 
be greater than anything known upon the surface of 
the earth. Some of the mountains would be 20,000 or 
30,000 feet higher than at present. It is not }Drobable 
that these mountains ever did extend to this elevation ; 
but rather, that as the rocks folded they were worn away, 
though not so rapidly as they were upfolded. The folding 
action was so slow, that the rock layers could be partially 
reduced and the elevation of the mountains thereby greatly 
lessened. Therefore, even before the folding of a mountain 
is finished, a large part of its mass may have been worn 
away by the agents of denudation (Fig. 229). 

Sculpturing of Mountains. — The carving of mountains is 




Fig. 226. 

Section across a mountain, showing normal extension of 

strata. 



PLATEAUS AND MOUNTAINS. 



365 



the result of an extremely complex series of actions, and it 
would be impossible to adequately treat the subject in so 
small a book. There is always a relation between rock 
structure and position ; and the mountain form is the result 
of the interaction of the forces of folding and of denudation, 
which operate differently according to the different positions 
and kinds of rocks. Some idea of the topography that 
results from this interaction may be obtained from the 
accompanying illustrations. (See also Chapter XXI.) 

The Drainage of Mountains. — The drainage of mountains is 
generally guided by the rock 
structure, or else by the rock 
position. Valleys are liable to 
be formed in layers of rela- 
tively soft rock, and streams 
are liable to have their courses 
guided by the ridges of the 
mountains. Therefore one of 
the characteristic features of 
mountain drainage is that 
of parallelism between moun- 
tain ridge and stream course (Fig. 227). The tributaries 
to these longitudinal streams, flow down the valley sides 
in direct courses ; and occasionally the streams cross the 
mountain ridges (Fig. 228) through deep and rather narrow 
gorges. It is possible that in some cases these transverse 
valleys are along the courses occupied by the streams which 
existed upon the country before the mountains began to 
form. Such are known as antecedent valleys, since they had 
their direction determined before the mountains began. It 
is believed that these streams were able to maintain their 
course across the growing mountains ; and if this really be 
so, it is another evidence of the extreme slowness of mountain 




Fig. 227. 
A bit of mountain drainage. 



366 



PHYSICAL GEOGRAPHY. 



growth ; for if mountains are folded no more rapidly than 
streams are able to cut their channels, then their growth 
must be remarkably moderate. Since there are other possi- 
ble explanations for these transverse valleys, we must con- 
sider this explanation as merely an hypothesis. 

Lakes are very 
common among 
mountains, their ori- 
gin in these places 
usually being the 
folding of the rocks^ 
which form dams 
across the stream 
courses. By this ac- 
tion of rock folding, 
streams may, in some 
cases, be transformed 
into lakes which 
maintain an outflow 
in the same direction 
which the river for- 
merly held ; or, in 
some cases, folding of 
the rocks may actu- 
ally turn the stream 
from its course, and 
make it begin to cut 
a valley at one side. Since the origin of these mountain 
lakes is that of rock folding, it very often happens that 
they are exceedingly deep. Generally their area is not 
great ; but there are some immense basins, the interior 
basins previously described, which have all the charac- 
teristics of lake basins, but which are prevented from being 




SCALE OF MILES 

1 I 1 I I I I 

12 3 4 5 6 

Fig. 228. 
Mountain drainage. 



PLATEAUS AND MOUNTAINS. 



367 



occupied by lake water because of 
the slight rainfall of the region in 
which they exist. 

Destruction of Mountains. — It has 
been said that mountains are the com- 
bined result of the folding of rocks 
and denudation. When they are 
growing, the action of folding ex- 
ceeds that of denudation, and the 
mountains continue to increase in 
elevation (Fig. 229). With this in- 
crease, stream action and the action 
of weathering have their power in- 
creased, and the mountains are very 
rugged. They are rugged partly 
because they are high, and partly 
because they are deeply carved 
by stream erosion. Therefore the 
highest and most rugged mountains 
in the world are the youngest; and 
among such mountains, lakes are 
usually present ; for the recent, or 
perhaps the present folding of the 
rocks has transformed a part of the 
streams into lakes. 

After the folding has ceased, there 
is no longer a tendency to become 
higher ; but the action of denudation 
still progresses uninterruptedly, and 
this tends to constantly lower the 
mountains, and, in the course of time, 
to render them less irregular. The 
lakes are removed, the mountain 



frS. 



a so w 



* £ 




,') 



-y 



368 



PHYSICAL GEOGRAPHY. 



peaks lose in elevation, the ridges are worn down, the streams 
have chosen the softer layers for their valleys, and the aspect 
of the mountains has become quite changed. This is the stage 
which has been reached by the Appalachians. These moun- 
tains were once much higher than now ; and since they have 
long been exposed to the destructive action of weathering 
and erosion, they have lost their ruggedness, and are strik- 




. MlatiS *'*«*_ 



Fig. 230. 
A mountain ridge in Colorado, showing hard layers etched into relief. 

ingly in contrast with such as the Rockies, the Himalayas, 
and the Alps, which are examples of young mountains. 

This action of destruction may be carried beyond the stage 
reached in the Appalachians, and whole mountain chains 
may be worn down to their very roots, and reduced to a 
series of relatively low hills. The highland portions of New 
England, New Jersey, and the entire region from this state 



PLATEAUS AND MOUNTAINS. 369 

to the Carolinas, east of the base of the Appalachians, repre- 
sents such an old mountain range. 

As a result of this mountain destruction, many interesting 
changes are brought about ; but the most striking result is 
the etching of the surface, so that everywhere the elevations 
are those of hard rocks, while the depressions occur in the 
soft strata. At first the mountain ridges may have had for 
their surface rock some soft layer which was bent up into 
a ridge (Fig. 262). But after long exposure to denuda- 
tion, the soft layers are worn down most rapidly, and the 
hard ones allowed to stand up (Fig. 230), so that there is 
this final result of relation between the rock structure and 
topography. This change may often go so far as to trans- 
form the old mountain valleys to mountain tops, and to 
wear down the original mountain ridges until they have 
become mountain valleys. Among the Appalachians there 
are numerous instances of this transfer of conditions ; and 
we then have represented what are known as synclinal 
mountains, the nature of which will perhaps best be under- 
stood by an examination of Fig. 229, E. 



REFERENCE BOOKS. 

Reade. — The Origin of Mountain Ranges. Taylor & Francis, London, 

1886. 8vo. 21s. 

For Structure of Appalachian Mountains, and an account of experi- 
ments in mountain folding, see Willis, Thirteenth Annual Report, U. S. 
Geological Survey, Washington, 1893. 

For structure of Basin Ranges, see Russell, Fourth Annual Report of 
the same, 1884. 
2b 



CHAPTER XX. 

VOLCANOES, EARTHQUAKES, AND GEYSERS. 

Volcanoes : Distribution. — Nearly all of the volcanoes of 
the earth are located either in the ocean or within a short dis- 
tance of the coast (Plate 27). They occur in lines, and are 
very commonly present in the highest mountains, although 
such systems as the Himalayas and the Alps furnish excep- 
tions to this. The mountains with which they are associ- 
ated are those in which there is a gradual growth at pres- 
ent in progress. In many cases they occur in archipelagoes 
near the coasts of continents. There is a line of recent 
volcanoes, along which there are many still in action, ex- 
tending from South America to Alaska : then crossing to the 
Asiatic coast, the line continues down to the East Indies. 
This is the most extensive volcanic belt of the world. 

The greater number of the volcanoes are now found in the 
Pacific or on the borders of this ocean. Though there are 
some in the Atlantic, this ocean is comparatively free from 
them. Along the mid- Atlantic ridge there appears to be a 
line of volcanic action, and some of the cones are still in 
eruption. Iceland and Tristan da Cunha are situated on 
opposite ends of this line, while the Azores, Canaries, and 
other islands are also in the belt. Volcanoes also occur in 
other parts of the earth, and there is reason to think that 
in some places they are present beneath the surface of the 
ocean. Indeed, volcanic cones have been known to rise 
above the sea, two instances of this being, one in the Medi- 
terranean and the other off the coast of Alaska. 

370 




Face page 370. 



Approximate distribution of active and recent volcanoi 



i wpm% 




annual isotherms of the waters of the ocean surface. 



VOLCANOES, EARTHQUAKES, AND GEYSEBS. 371 

In the United States, excluding Alaska, there are now 
no volcanoes which are known to be in eruption. Both in 
Alaska and in Mexico there are active cones ; and in the 
northwestern part of the country, in the state of Washington, 
there are some whose form is so perfect that they may still 
be active volcanoes in a dormant condition. Indeed, there 
are reports that some volcanoes in the far west have been in 
eruption since the region was inhabited. While at present 
there is very little if any volcanic activity in this country, 
the Cordilleras of the west have just passed from a period 
of most remarkable volcanic action. There are thousands 
of cones on the plateaus and in the mountains of this region, 
some of them perfect in form, as if still in action, others 
the nearly destroyed remnants of cones. 

In other parts of the world there are regions in which 
there are now no volcanoes, but in which there has been 
much volcanic action during the past geological ages. This 
is true of the Auvergne region of central France, of the 
British Isles, of the east coast of the United States, and 
many other places. On the other hand, there are areas of 
the earth in which volcanoes are not only now absent, but 
from which they have always been absent since the begin- 
ning of the Cambrian time. This is true for most of the 
plains of the Mississippi valley. 

Materials Erupted. — Steam is perhaps the most important 
of substances emitted from volcanic vents (Fig. 231). This 
is important not merely because it occurs in vast quantities, 
but also since it is the immediate cause for the volcanic 
eruption. Of solid materials there are two important classes, 
the lava, which reaches the surface as molten rock and then 
cools, and the volcanic ash or pumice, which is really lava 
blown full of holes and made light and porous. The pumice 
is made into this form by the expansion of the steam which 



372 



PHYSICAL GEOGRAPHY. 



was imprisoned within it while the molten rock existed 
beneath the surface of the earth. Besides these, there are 
less notable quantities of other substances, chiefly certain 
gases, such as hydrogen, chlorine, sulphurous gas, etc. 

Some of the steam passes into the air as vapor, but much of 
it falls to the earth near the volcano, producing very heavy 
rains, and often causing deluges in the neighborhood of the 
cone. During an eruption there are often violent thunder 

storms, in which 
the rain is largely 
derived from this 
source. When the 
water falls upon a 
cone whose surface 
is strewn with vol- 
canic ash, the tor- 
rents of water 
wash this loose 
material down the 
hillsides, and a 
great mud flow is 
produced. These 
are often very de- 
structive, and it was such a flow as this which buried the 
city of Pompeii during the eruption of 79 a.d. (Fig. 236). 
The mud flowed over the houses, entered cavities, and 
formed casts of objects, thus protecting them from destruc- 
tion, so that in the excavations which have been made during 
the present century, we have obtained very perfect records 
of the conditions under which the Romans lived 1800 years 
ago. 

The lava flow reaches the surface as a mass of liquid rock, 
and passes down the side of the cone, often extending 




Fig. 231. 

Vesuvius in eruption, 1872. 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 373 



HT- 



far beyond the base and deluging the country over which 
it passes. It advances first as molten rock, then a slight 
crust forms over it, and its motion becomes relatively slow. 
Toward the last of the eruption, the lava is covered with such 
a thick crust of rock that one may walk upon its surface, 
although at the depth of a few feet there is still molten 
lava. The surface of such flows is extraordinarily rough ; 
for as the liquid part moves, the solid crust is often broken 
into fragments (Fig. 232). In 
some rough-surfaced lava flows, it 
is almost impossible for a person to 
travel over the lava boulders. 

The lava does not extend to a 
very great distance from the place 
of ejection, for the flows are rarely 
more than 20 or 30 miles in length. 
Therefore the effect of a lava flow 
is relatively local. In some places, 
as for instance in the Snake River 
valley of Idaho, and in other parts 
of the plateau region of the west, 
lava has reached the surface through 
great fissures. Instead of building 
up a cone it has welled out and 

spread over the surface, filling valleys, and often submerg- 
ing hills, over areas of thousands of square miles. In places 
the lava fills the valleys to the depth of 2000 or 3000 feet. 

During an eruption in which ash is sent to the surface, 
these light rock fragments are often ejected to great heights 
in the air, in some cases apparently reaching elevations of 
several miles above the surface. The heavier fragments fall 
back upon the cone, or in its immediate neighborhood ; but 
many of the lighter fragments are sent so high into the air, 




Fig. 232. 

Surface of a recent lava flow 

in the west. 



374 



PHYSICAL GEOGRAPHY. 



that before they have been able to fall, they are blown by the 
wind currents to a considerable distance from the cone. 
In the very violent eruption of Krakatoa, in the Straits of 
Sunda (in 1883), the liner particles of volcanic ash extended 
so high into the air that they did not entirely reach the 
earth for a year or two. It is estimated that the fragments 
reached a height of 50,000 feet ; and this ash in the upper 

layers of the air, 
drifted over the earth 
in the prevailing cur- 
rents, causing bril- 
liant sunsets in both 
Europe and America. 
Since volcanoes are 
largely located either 
in or near the sea, 
much of the ash that 
is erupted, falls upon 
the surface of the 
ocean and drifts 
about ; for pumice is 
so light that it will 
float upon water. 
After the eruption 
of Krakatoa, vessels 
sailing in the region of the East Indies, often encountered so 
much floating pumice that sailing was difficult. Some of 
this is stranded upon the coast and broken into small bits of 
sand, but much of it drops to the bottom of the ocean ; for 
the pumice either decays and breaks into fragments, or else 
becomes waterlogged and sinks to the bottom. 

Eruptions of Volcanoes. — There is a great difference in. 
the kind of eruption in different volcanoes, and even at dif- 




Fig. 233. 

Lake formed by a lava dam, to be seen in the 

background. 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 375 

ferent times in the same cone. On the Lipari Islands, of the 

Mediterranean, there is a small volcano which is in almost 

constant action 

(Fig. 234). The 

eruptions are of 

ash, and the 

violence is not 

great, so that 

sailing vessels 

may pass by the 

island without 

danger. So far 

as the history 

of these islands 

is known, there f-\- 

have been no ; - ; j » v ^' 

real destructive Fig. 234. 

eruptions. In 

the case of Krakatoa, on the other hand, there has been but 

one eruption during the present century. In the spring of 




r - . 






" l 


K ■■■■• ■ 
P ■ 




'-'■"-!», 




L**™-««*^S 




■ ^ 


r^rr ~- 


[ ■ - : . ■■-'■■■■ ■ 










Fig. 235. 
Diagram showing the disruption of Krakatoa. 



376 



PHYSICAL GEOGRAPHY. 



1883 there were signs of activity in the volcano, and these in- 
creased until August, when occurred the most remarkable 
eruption of recent times. One half of the cone was entirely 
blown away (Fig. 235) ; and where the high volcanic island 
existed, there is now deep water in place of a part of the 
island. There are numerous other instances of violent erup- 
tions, and in 
Iceland these 
are not at all 
uncommon. 

Many vol- 
canoes have 
violent erup- 
tions at one 
time, and then 
moderate ac- 
tion. This 
was the case 
with Vesuvi- 
us, which was 
not in erup- 
tion from the 
time of the 
first occupa- 
tion of Italy, 

until the year 79 a.d. (Fig. 236). Then an explosion took 
place which was the most vigorous that has been experienced 
in the recorded history of the cone. A very considerable part 
of the old mountain, which was known as Monte Somma, was 
blown away, and a number of towns were destroyed, includ- 
ing Pompeii and Herculaneum. Since then, Vesuvius has fre- 
quently been in eruption, but none have equaled that of 79. 
Ash-erupting volcanoes are usually more violent than 




Fig. 236. 

Vesuvius, from Pompeii. Monte Somma on the right, in the background. 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 377 

those which send forth lava. Of the latter kind, the volca- 
noes of the Hawaiian Islands furnish excellent illustration. 
Here one may stand on the margin of the crater and look 
upon a great lake of molten rock. The surface of this 
lake gradually rises; and, after several years, a lava flow 
breaks through the side of the cone and flows down toward 
the base, while at the same time the surface of the lava 
lake rapidly descends. The eruption is not from the crater, 
but through fissures that are broken in the side of the cone. 
The activity of these volcanoes is never excessive. 

The most violent volcanoes are those in which there are 
the longest periods of rest between eruptions. The tube 
through which the lava escapes becomes filled with solid 
rock, and this appears to act in a measure like the closing of 
the safety valve of an engine. The steam, which is the 
immediate cause for the eruption, finally accumulates suffi- 
cient force to blow out the plug, or else to blow away a part 
of the cone. 

Volcanoes might be divided into three groups upon the 
basis of their condition. Some are active, and their periods 
of eruption are variable, in some cases being many years, 
in others only a few years, or even less than a year apart 
(Figs. 231, 234-236, and 239). A second group is that 
of the dormant volcanoes, in which there is no present 
sign of activity, but which at any time may break forth 
in eruption (Fig. 238). Vesuvius was a dormant vol- 
cano, and the inhabitants of the region believed it to 
be free from eruption; for towns and vineyards dotted 
the slopes of the mountain when it began to break 
forth in the year 79. After this long period of rest, the 
length of which cannot be estimated, but which certainly 
covered several centuries, Vesuvius became an active vol- 
cano, and has maintained this condition ever since. After 



378 



PHYSICAL GEOGRAPHY. 



awhile any volcano will cease action permanently, and then 
it becomes extinct (Fig. 237). The lesson taught by Vesu- 
vius and Krakatoa, should lead us to include in this group 
only those volcanoes which have been quiet for so long a 
time that there is almost no possibility of eruption. It is 
possible that some of the supposed extinct volcanoes of the 
far west are really dormant (Fig. 238). 




Mt. Hood- 



Fig. 237. 
an apparently extinct volcano. 



Form of Cone. — When a volcano first begins to form, an 
opening is made in the ground, through which ash and lava 
are emitted, together with steam and other gases. The 
accumulation of the ejected materials soon builds a cone 
around this orifice. A single eruption will suffice to form a 
cone, the reason for the conical shape being, that the greatest 
quantity of material accumulates nearest the place of ejec- 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 379 



tion (Fig. 234). With successive eruptions the cone grows 
higher ; and if they continue through the same opening, there 
is produced at the top, and in the center of the cone, a crater 
which leads down into the interior (Fig. 234). 

If weathering and erosion were not present to destroy the 
conical form, in volcanoes that emit ash we would have pro- 
duced a very per- 
fect cone, whose 
angle of slope 
would be as great 
as that assumed 
by gravel when at 
rest. It is proba- 
ble that this is 
approximately the 
form of the cone 
which is built be- 
neath the surface 
of the ocean, where 
there is no action 
of denudation. 
On the land there 
is constantly a 
tendency to re- 
move the materials 
which are building the cone. Instead of a slope equal to 
that of a gravel bank, the angle is lessened by the washing 
action of rain, and the cone is gullied by stream valleys. 
In some cases, where ash is ejected in great quantities and 
frequently, the angle of slope is high, and the form of the 
cone quite perfect. In some of the sharpest cones the angle 
of slope is as great as 25° or 30°. This is illustrated in 
Popocatapetl in Mexico, and in Fusiyama (Fig. 239). 




Muir's Butte, California — a volcano recently in 
eruption. 



380 



PHYSICAL GEOGRAPHY. 



Violent eruptions tend to destroy the perfection of the 
cone; and in the case of Krakatoa, the volcano was divided 
into two parts, one of which disappeared into the air (Fig. 




Fig. 239. 
Fusiyama — a Japanese volcano. 



235). The same is true of Vesuvius, and a part of the old 
rim which formed Monte Somma was blown away ; and 
now Vesuvius, as viewed from Pompeii, shows a perfect cone 




Fig. 240. 

Angle of slope of volcanoes, a, extremely steep ash cone (approximately repre- 
sented in Fusiyama and in submarine volcanoes) ; b, lava cone (Hawaiian 
Islands). 

partly surrounded by a mountain wall, which is the remnant 
of old Somma (Fig. 236). 

The eruption of lava produces a very much flatter cone. 
This is well illustrated in the Hawaiian Islands, where, 



VOLCANOES, EARTHQUAKES, AND GEYSEBS. 381 

although the volcanoes are exceedingly high, the slope is 
quite moderate, being less than 10° (Fig. 240). This is due 
to the fact that lava tends to flow away as water does, and 
consequently to broaden the cone as well as to lessen the 
slope. Many volcanoes are at one time erupting ash and 
then lava ; and the cone produced is intermediate in form 
between these two extremes. Such are Vesuvius and iEtna, 
and indeed the majority of the volcanoes in the world. 

Effects of Volcanic Eruptions. — One of the most important 
effects of eruptions is the addition of rock material to the 
surface from underground sources. An appreciable part of 
the rocks of the crust have been produced in this way. 
Volcanic action also furnishes heat to parts of the earth, 
especially where rocks are injected ; and this is one of the 
causes for hot springs, for many mineral veins, and for 
the metaniorphism of some rocks. The lava flows also in- 
terfere with the drainage of streams, sometimes damming 
them and forming lakes (Fig. 233), at other times occupying 
valleys and causing the streams to begin the work of forma- 
tion of new gorges. 

When eruptions occur in the ocean, great waves are pro- 
duced, which sweep upon neighboring coasts, and often 
cause vast destruction of life. In the East Indies, the low- 
lying coasts are frequently subjected to this danger (see pp. 
178, 179). Earthquakes are also produced as a result of 
volcanic eruptions ; and both by this indirect means, as 
well as by the lava and ash from the eruption, the destruc- 
tion of human and animal life is often very great. It is 
estimated that over 50,000 lives were lost during the erup- 
tion of Krakatoa. Practically every vestige of life was 
extinguished from the island, and the destruction extended 
to neighboring islands. 

Extinct Volcanoes. — When a volcano has ceased action, 



382 



PHYSICAL GEOGRAPHY. 



the forces of denudation seize upon the cone and wear it 
away. At first the regularity of the cone is destroyed by 
the gullying action of streams (Figs. 237 and 241), then 
its size decreases, and finally merely a remnant of it is left. 
This remnant is always that of the central part of the cone, 
partly because this is the divide and hence less exposed to 






Fig. 241. 
Mt. Shasta on the left ; Shastina, a more recent cone, on the right. 



erosion, but mainly because it is the place where the hardest 
rock occurs. The old vent or tube of the volcano is filled 
with rock from the last eruption that has occurred ; and since 
this is less porous than the lava or ash that forms the cone 
itself, it is much more resistant to weathering. These necks 
or plugs (Fig. 242) of volcanoes are present in all regions 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 383 




Fig. 242 

Mato Tepee, Wyoming - 
neck. 



u&faMC*^ 



■ an old volcanic 



where volcanic action has recently ceased. Upon the western 
plateau there are thousands in all stages of destruction. 

As the volcano disappears, denudation reaches places into 
which lava has been intruded in the form of dykes or bosses; 
and when these are harder than the surrounding rock, they 
stand up as ridges. With 
the wearing away of the 
surface, the lava flows also 
disappear ; and where they 
are harder than the rocks 
upon which they rest, they 
often protect these from 
destruction, causing flat- 
topped hills and small 
table-lands. These lava- 
capped buttes or mesas 
(Fig. 218) are very com- 
mon in the regions be- 
tween the Rocky Mountains and the Pacific coast. 

Cause of Volcanoes. — The immediate cause of volcanic 
eruptions is the presence of steam ; and in a measure the 
eruption may be compared to the bursting of a boiler. 
There is steam present in a superheated condition, this tends 
to find relief, and the eruption occurs. The origin of the 
heat which causes the melting of the rock cannot be stated. 
It has to do with the heated condition of the earth, and 
since we are not certain just what this condition is, we of 
course are not able to state what causes the molten rock. 
The same cause that produces the folding of mountains 
appears to operate in the formation of volcanoes ; and the 
volcanic action is in most cases, if not in all, an indication 
that the crust is folding. 

Earthquakes. — By far the greater number of earthquakes 



384 



PHYSICAL GEOGRAPHY. 




Fig. 243. 

The earthquake wave. E, epicen- 

trum. F, focus. 



j., • _•, 



occur either near volcanoes or among mountains, though 
some have occurred at great distances from either of these. 

The earthquake is a jarring 
of the rocks, caused by some 
shock which is transmitted as a 
series of spherical waves in all 
directions through the strata. 
The point of origin of the 
shock is known as the focus 
(Fig. 243), and from this cen- 
ter the earth waves move in all 
directions. If the rocks were of uniform texture, the earth- 
quake waves would have - 

a spherical form; but /: , ■" Z" ->"--*. - .>-' v ~._W ! . '^v 
since the strata vary in 
character, the rate of mo- 
tion differs, and conse- 
quently the spherical 
form is distorted (Fig. 
244). 

The point on the earth's 
surface directly above the 
focus is known as the 
epicentrum, and this is 
the place where the 
shock first reaches the 
surface. The waves come 
from the "earth at equal 
distances from this point, 
and on all sides of it. 
If the rock texture were 
uniform, the shock would 
be felt at the same time at all points whose distance from the 



M, 






7 



Fig. 244. 
Earthquake waves of Charleston earthquake, 
showing effect of folded rocks of Appala- 
chians. 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 385 



epicentrum is the same. The most violent part of the 
earthquake is in the immediate vicinity of the center, while 
it decreases quite uniformly away from this (Fig. 245). 

Even during violent earthquakes, the amount of movement 
of the rocks is not very great; but the effects of the jar are 
often very disastrous. Parts of cliffs are thrown down, 
landslides produced, 
houses destroyed (Fig. 
246), trees overturned, 
and general destruction 
caused. The destruc- 
tion of human life is 
greatly increased by the 
fact that houses are 
readily thrown down 
by earthquake waves. 
When earthquake 
shocks occur in the 
ocean, great sea waves 
are often produced, and 
these, sweeping upon 
the coasts, devastate the 
lowlands. 

Any jar in the earth 
will produce an earth- 
quake. During the ex- 
plosion of dynamite at Hell Gate, near New York, a few 
years ago, a shock was started which was measured as 
far away as Washington on the one side, and Boston on the 
other. The great earthquake shocks are evidently connected 
either with volcanic eruptions or with faulting in the rocks. 
The violent eruption of a volcano, like that of Krakatoa, 
sends a series of earthquake waves through the rocks ; and in 
2c 




Fig. 245. 
Earthquake shock in Japan. 



386 



PHYSICAL GEOGRAPHY. 



the time immediately preceding volcanic eruptions, earth- 
quake shocks are very common, being apparently the result 
of unsuccessful efforts of the lava to force its way to the 
surface. As the rocks are broken apart, each step in its prog- 
ress toward the surface produces a jar. When mountains 




Fig. 246. 
Effect of earthquake in Japan, 1891. 

are being formed, rocks are often broken and faulted; and as 
they break and slip, waves are started which produce earth- 
quake shocks (Fig. 247). Many of the most violent earth- 
quakes of the world appear to be attributable to this cause. 

Geysers and Hot Springs. — Underground water, after a 
passage through the earth, often finds its way back to the 
surface in a heated condition. In such cases hot springs 
are produced, and these are generally mineral springs ; for 



VOLCANOES, EARTHQUAKES, AND GEYSERS. 387 



hot water, in passing through the crust, finds many mineral 
substances which it can dissolve (Fig. 105). Hot springs 
are quite commonly found in association with volcanoes; 
and it is very probable that the heat of the water is in most 
cases furnished by some supply connected with volcanic ac- 




Breaking of the earth along the fault line which caused the Japanese 
earthquake shock of 1891. 

fcion. Even after volcanoes have ceased activity, hot springs 
may remain in the neighborhood. 

Sometimes hot springs have the peculiar habit of bursting 
forth into eruptions of steam and hot water, and then a geyser 
s produced (Fig. 249). A geyser may be defined as a hot 
pring which has a habit of intermittent eruption. One of 
;he geysers of the Yellowstone Park region, the Artemesia, 
rvas for a long time known as a hot spring, and then suddenly 



388 



PHYSICAL GEOGRAPHY 



began eruptions like the other geysers of the Park. While 
hot springs are very widely distributed, geysers are quite 
uncommon. There are only three places in the world where 
they are features of importance, one being the Yellowstone 
Park, the second in Iceland, and the third in New Zealand. 
In all of these cases, the geysers are bringing to the surface 




Fig. 248. 
Crater of Oblong Geyser, Yellowstone Park. 



large quantities of chemically dissolved mineral matter ; 
and in the Yellowstone region, craters are built around the 
geyser (Fig. 248). 

There is much difference in the time between the eruptions 
of geysers, some being in eruption every few hours, others hav- 
ing very irregular periods of action. Hot water slowly boils 
in the tube, then it overflows gently, and suddenly, with very 



VOLCANOES, EARTHQUAKES, AND GEYSEBS. 389 

little warning, bursts forth into eruption, when the air is 
filled with a great column of hot water and steam, which in 
the case of the larger gey- 
ser usually rises to a 
height of 100 or 200 feet 
(Fig. 249). 



REFERENCE BOOKS. 

VOLCANOES. 

Dana. — Characteristics of 
Volcanoes. Dodd, Mead & 
Co., New York, 1891. 8vo. 
$ 5.00. (A very complete and 
valuable discussion of the sub- 
ject.) 

Hull. — Volcanoes: Past and 
Present. Scribner, New York 
(Contemporary Science Se- 
ries), 1892. 12mo. $1.25. 

Judd. — Volcanoes. Appleton 
& Co., New York (Inter- 
national Scientific Series), 
1881. 12mo. $2.00. 

For Eruption of Krakatoa, see The Eruption of Krakatoa (edited by 
Symons). Trubner & Co., London, 1888. 4to. 30s. 

Eor Hawaiian Volcanoes, see Button, Fourth. Annual Report, U. S. 
Geological Survey, Washington, 1884. 

earthquakes. 

Milne. — Earthquakes. Appleton, New York, 1891 (International Scien- 
tific Series). 12mo. $1.75. 
For a description of the Charleston Earthquake of 1886, see Dutton, 

Ninth Annual Report, U. S. Geological Survey, Washington, 1889. * 

1 Nearly all of these articles in the U. S. Geological Survey Reports are well illustrated ; and 
since many of them are readily obtained free of cost, they should be widely used. 




Fig. 249. 
Old Faithful Geyser, Yellowstone Park. 



CHAPTER XXL 

THE TOPOGRAPHY OF THE LAND. 

General Statement. — Land forms are of two kinds: (1) those 
that have been built by some agency and (2) those that 
have resulted from the combined action of building and 
carving. By far the greater number of land forms are of 
the last origin, and there are few that have resulted exclu- 
sively from constructive action. There are two sets of 
forces working upon the earth in an effort to modify its 
surface : the one internal, which tends to make the surface 
diverse, the other mainly external and tending to level. As 
a result of the action of the former, the earth's surface is 
thrown into a series of waves, great and small, and some of 
these are even now in process of formation. 

If nothing had interfered, these earth waves would have 
made the surface very irregular, and the mountain chains 
would have risen to vastly greater heights, and often with 
much steeper slopes than we really find. In opposition to 
this force there are the agents of denudation, which derive 
their power chiefly from causes outside of the earth itself, 
and are mainly manifestations of solar energy, combined with 
complex causes, some of which are described in the first 
chapters of the book. By removing materials from the higher 
parts and spreading them over the lower areas, the agents 
of denudation are engaged in the work of leveling. In 
the course of this, it is often necessary, or most easy, to 
temporarily increase the irregularities, as is done by the Col- 
orado in its work of valley formation in the great Arizona- 

390 



THE TOPOGRAPHY OF THE LAND. 



391 



Utah plateau (Plate 28). The present land form is the 
result of the complex interaction of these forces, and it is 
still in process of change. 




Plate 28. 
Brink of Marble Canon, Colorado River. 



392 PHYSICAL GEOGRAPHY. 

Some parts of the earth are now being built up, others are 
being worn down by one cause or another. As a result of 
this, the surface of the earth presents most complex features ; 
but if we look at the causes and influences that are at work, 
it becomes a much more simple task to account for them. 
These may be briefly summed as follows : The crust of the 
earth is in movement, in some places upward, in others down- 
ward, here by broad uplift or downsinking, there by the more 
local and intense upfolding or downfolding which accompa- 
nies mountain growth. Some regions are therefore nat- 
urally high, others low ; some are mountains, others plains, 
and still others plateaus. Denudation is everywhere at 
work ; and since the conditions are variable, the results are 
quite different. Its action upon plains differs from that 
upon plateaus ; and in regions of horizontal strata, its effect 
is quite different from that produced when the rock position 
or attitude is complex. Not merely does the difference in 
rock position produce a perceptible effect, but the variations 
in resistance to weathering and erosion are of most funda- 
mental importance. These agents of denudation are also 
engaged in the work of construction ; for the materials 
taken from one place find rest in another, and often the two 
processes of tearing down and building up overlap. 

Constructive Land Forms : By Internal Forces. — It is to 
be borne in mind that in nearly every part of the land, no 
matter what the origin of the surface features, there is evi- 
dence of the action of the destructive denudation ; and there- 
fore in this section we deal merely with the skeleton, not 
with the perfected form. The larger diversities of the 
earth's surface, although greatly sculptured, owe their main 
features to the action of contraction of the earth's interior. 1 

1 Accepting the contractional hypothesis, as we may fairly do, for a work- 
ing hypothesis. 



THE TOPOGTtAPHY OF THE LAND. 393 

Thus the continents and mountains, considered without ref- 
erence to details, are true constructional forms, being built 
by the folding of the roots. In the same way, many of the 
plateaus, such as those which lie at the base of the Rocky 
Mountains, are due to the elevation of a part of the earth's 
crust; and many plains have also been given their present 
condition by land movements. This is the case with the 
coastal plain which forms the eastern margin of the country 
south of New York. This represents an old, nearly level 
sea bottom, very recently raised into the condition of land ; 
and another elevation of this part of the continent to a 
height of 600 feet, would add a plain which in some places 
would be more than 100 miles in width. 

The volcano is also a constructional form dependent upon 
the heated condition of the rocks beneath the crust (Figs. 
234-241). It is built up and is formed into a typical topo- 
graphic feature ; but under different circumstances this form 
varies somewhat. The cone results from the piling up of 
materials derived from beneath the crust, and accumulated 
into a conical heap around the place of ejection. Partly 
because of denudation, and partly because of the explosive 
action of some eruptions, the cones are much less perfect than 
they normally tend to be. 

By Agents of Denudation. — Some topographic features are 
produced directly by the building action of the agents of 
denudation ; but these are usually of minor importance. 
As a cliff crumbles away, talus deposits accumulate at its 
base, and these often produce great sweeping slopes at the 
foot of steeply rising mountains (Figs. 118 and 219). Some- 
times this curve unites with that caused by denudation, and 
a double curve is then produced. The wind often blows 
sand into mounds, and these may cover great areas, com- 
pletely burying the underlying topography. These are par- 



394 PHYSICAL GEOGRAPHY. 

ticularly liable to be formed near seacoasts (Fig. 120); but 
sand-dune areas are also common in arid regions. 

When filled with sediment, and transformed to swamps or 
plains (Fig. 172), constructional forms of monotonous regu- 
larity are often built in the site of lakes. The same condition 
results when lakes are displaced by other causes, as is the case 
when they evaporate ; and many of the great alkaline plains 
or fiats of the Great Basin are old lake bottoms (Fig. 150). 
The disappearance of a glacial lake often leaves an exten- 
sive plain, as is so well illustrated in the great wheat plains 
of the valley of the Red River of the North (Fig. 215). 
In these cases the shore lines are also left, and these topo- 
graphic forms, though of minor importance, are often strik- 
ing features in the landscape (Fig. 170). Deltas (Fig. 
154), bars (Fig. 213), and spits (Fig. 196) are built up in 
the lake waters ; and upon the disappearance of the lake 
these are left upon the valley sides (Fig. 170). 

Rivers also build deposits, the most notable being deltas 
and floodplains ; but in somes cases, terraced valley sides 
result from the constructive action of the river floods. One 
of the most important causes for the details of the topog- 
raphy in northern United States, is found in the recent 
glaciation ; and much of this topographic variety is due to 
the building action of the ice. With the debris that it car- 
ried, the glacier formed great plains, either by direct depo- 
sition from the ice, or in a secondary way through the 
intervention of water produced by ice melting. Much of the 
prairie country of the Central States owes its present levelness 
to these causes. In other places, hills of peculiar and irregu- 
lar form were built by the ice. To the majority of people who 
live in the glaciated belt, these hills of gravel and unstrati- 
fied till must be familiar features; and in the morainal regions 
they are strikingly developed. (See Chapter XVII.) 



THE TOPOGRAPHY OF THE LAND. 395 

The ocean is the great receiving ground for the waste of 
the land ; and for the most part the debris is spread quite 
evenly over the bottom, producing a plain, which in some 
cases is partly raised above the sea. But along the shore 
line, the constructive action of the ocean is producing many 
irregularities, though here, as elsewhere, the actions of tear- 
ing down and building up ar so intimately associated that 
it is often difficult to draw th line between them. Still, 
the beaches (Figs. 200 and 201), the bars, the long sandy 
islands (Fig. 194), and other similar coastal features, are 
often mainly the result of the action of the waves and cur- 
rents in building up materials furnished by various means. 
When, for any reason, the level of the sea is changed in its 
relation to the land, these shore-line formations are either 
submerged, or, if the land rises, are left as ancient shore 
lines, which then resemble those remaining when lakes dis- 
appear. 

By Animal and Plant Life. — In various ways, both animals 
and plants are engaged in constructing land forms. The 
salt marsh of the seashore (Fig. 206), and the swamps of 
the land, in part represent this action ; but the most notable 
action of life in this respect is that of the corals, which are 
building reefs (Fig. 207). It is true that the corals do not 
build the reefs above the sea level ; but a slight elevation of 
the bottom has oftei raised them to the air. Also, the 
action of the waves may pil the coral fragments above 
the reach of ordinary waters ■ and wind action, by blowing 
the coral fragments into dune-like hills, then causes them to 
rise still higher. By these constructive processes combined, 
many islands are built in the sea (Figs. 208 and 209). 

Effect of Rock Structure upon Topography. — The land forms 
constructed in the ways above described, are subjected to 
attacks from all the agents of denudation ; and as a result 



396 



PHYSICAL GEOGRAPHY. 



of this, the land surface presents many diversities. Under 
uniform conditions, denudation affects rocks differently 
according to (1) their elevation, (2) their position, and 
(3) their structural features. Moreover, the intensity of 
denudation varies ; and as a result of these facts, land forms 
differ from place to place. It ; s impossible here to enter into 
this subject in any considerable detail ; but some of the 
main principles may be briefly stated. 

Much depends upon the ease with which materials may be 




Fig. 250. 

View in Brazil, showing hard layer etched into relief by the removal of the less 
resistant enclosing rocks. 

removed (Fig. 250). In high mountains, where the grade 
is steep and d udation intense, the etching of the rocks is 
very sharply done (Fig". 224, 225, 230, and 261); and hence, 
in such places, we have J '\e characteristic ruggedness of high 
mountains ; but when the mountains are low, even though 
the difference in rock hardness may be great, the outlines 
are less angular and more rounded and flowing. In this 
connection one may contrast the Alps (Figs. 143, 144, and 
220) with the Highlands of Scotland, or the Rockies of 



THE 10P0GRAFHY OF THE LAND. 



397 




Plate 29. 

Navajo Church, Arizona, showing sharpness of denudation in an arid region. 
Soft clay, capped hy harder rock, in foreground. 



398 



PHYSICAL GEOGRAPHY. 




Colorado (Figs. 221 and 223) with the Appalachians or the 
Adirondacks (Figs. 263 and 264). 

With conditions of aridity, the soil covering is readily 
removed from the rocks, so that they are exposed to the air ; 
and hence, here also, angularity and ruggedness of topography 
prevail (Figs. 122 and 142 and Plates 28 and 29). Often- 
times the streams cannot carry the material furnished to 

them, and instead 
of trenching the 
highlands, they 
flow on the sur- 
face of a plateau. 
This is the case 
with the river 
Platte. The in- 
tensity of denuda- 
tion is therefore 
of great impor- 
tance, and this 
varies with the 
stage of develop- 
ment, so that there 
is an intimate 
relation between 
topography and 
the age of topo- 
graphic forms. 
The young valley is a sharply denned feature (Fig. 133), 
while the mature valley, in which the intensity of erosion 
has ceased, is rounded under the more widespread, but more 
moderate action of weathering (Fig. 135). Altitude is an 
important element in this connection, but it is by no means 
the only one. 




Fig. 251. 
A cliff in the Yosemite. 



THE TOPOGRAPHY OF THE LAND. 



399 



Much depends upon the rock structure. Even though it 
be soft, a rock of uniform texture produces massive effects. 
The granite of the Yosemite (Figs. 164 and 251) is com- 
posed of materials uniformly arranged, — hence the bold, 
regular outlines. Massive beds of limestone produce the 
same effect ; and among some of the ranges of the Rocky 
Mountains, where the rock is a thick bed of limestone of 
quite uniform texture, there are places of great precipitous- 




Fig. 252. 
Cliffs in the loess clay of China. 



ness. Even when the surface is covered with consolidated 
clay, this uniformity impresses itself upon the topography, 
as is so well illustrated in the Chinese region (Fig. 252). 
Upon the seashore these massive rocks are often cut into 
cliffs, which frequently rise to great heights, as in the case of 
the chalk cliffs of England. 

On the other hand, if the rock is in layers, or if for any 
other reason it is rendered mechanically weak in places, the 
boldness disappears. On the seacoast, the weakness of the 



400 



PHYSICAL GEOGRAPHY. 



rocks is taken advantage of by the waves, and the weak 
places indicated by an indentation in the coast. Where the 
rocks are jointed or broken, or where one layer is softer than 
another, sea caves (Fig. 198), chasms (Figs. 199 and 253), 

and even small bays, may be 
I produced. The cliffs are not 
so high nor so angular as in 
the massive rocks (Figs. 254 
) and 255) ; for, both by the waves 
and by weathering, they are 
caused to crumble and to as- 
sume a more gentle slope. 

Since hard strata (meaning 
those resistant to denudation) 
are worn down with much less 
rapidity than soft ones, where 
these alternations exist in such 
a position as to be exposed to 
denudation, there is much ir- 
regularity introduced. Accord- 
ing to the attitude of the rocks, 
there is much variety in the 
topography. If the strata are 
horizontal, the hard layers tend to remain ; and between 
the rivers, there are relatively flat-topped hills, capped by 
these hard rocks. Their margins are steeply sloping, but 
the slope decreases where the layers are soft (Figs. 256 
and 257). These features of the land are particularly well 
developed in arid regions, where differences in rock hardness 
are always etched with greater intensity than in moist coun- 
tries; and in such places terraces are often produced. These, 
which have been called terraces of differential degradation, 
are flat-topped where hard layers exist, while between two 




Fig. 253. 

Rafe's Chasm, Cape Ann, Mass 
wave-worn chasm in granite 



THE TOPOGBAPHY OF THE LAND. 



401 




Fig. 254. 
A rugged coast in massive granite, Cape Ann, Mass. 

such areas there is a steep ascent. In such a place, in 
traveling across country, one passes over a series of steps 
on the land (Fig. 217 and Plate 28). Such topography 




Fig. 255. 
A granite coast where the rock is much jointed, Gape Ann, Mass. 
2d 



402 



PHYSICAL GEOGRAPHY. 



is typical of plateaus, and particularly of those in arid lands. 
On the seashore, the tendency to produce a step-like coast 
exists where the horizontal rocks outcrop in cliffs composed 

of layers of different hard- 
ness. 

With gently dipping 
rocks, very nearly the same 
kind of topography is 
produced ; but the flat- 
topped areas are less dis- 
tinct. In passing across a 
country in which the differ- 
ences in hardness of slightly inclined rocks are well brought 
out, as in the central part of Texas, the aspect of the country 
changes entirely, according to the direction pursued. If one 




Fig. 256. 

Effect of hard layers (unshaded) in the 
denudation of nearly horizontal strata. 








Signal Butte, Texas. An outlying hill protected by a hard cap of horizontal rock. 

travels at right angles to the dip, he may pass for long dis- 
tances upon a flat-topped terrace, bounded on one side by a 
steeply rising face, and on the other by a steeply descending 



THE TOPOGRAPHY OF THE LAND. 



403 



slope (Fig. 258). If going in the direction of the dip, one 
ascends a steeply sloping hill, then passes over a bench to 

H 





Fig. 259. 
H, hard stratum. 



Fig. 258. 

Step topography in region of inclined strata. H, H, H, hard layers; S, S, S, 

soft. 

another sloping hill, and this may be repeated many times. 
If the journey 
is in the oppo- 
site direction, 
there are a se- 
ries of descents 
with interme- 
diate terraces. 
Looking in the 
direction of the 

dip, one sees a series of hills, while the view in the opposite 

direction is over 
the surface of 
the plain. The 
flat areas are 
determined by 
hard layers, and 
the steep slopes 
are also due to 
their presence ; 
for they serve 
to protect the softer underlying layers from destruction. 
Where such a series of rocks occurs on the seacoast, the 




Fig. 260. 
H, hard stratum ; S, soft. 



404 



PHYSICAL GEOGRAPHY. 



form of the coast differs entirely according to the direction 
of the dip. If the waves beat against a series of rocks dip- 
ping toward the sea, they produce a gently-sloping shore, 
whose form and position are determined by a hard layer 
(Fig. 259). On the other hand, if the dip is away from the 
sea, the waves beat against a bluff (Fig. 260.) 

When the strata are inclined at a high angle, the hard 




Fig. 261. 
A ridge of hard rock etched into relief by more rapid removal of softer strata. 

layers tend to stand up above the surrounding country in the 
form of ridges, while the position of the softer strata is indi- 
cated by valleys (Figs. 230 and 261). These peculiarities 
are particularly well illustrated among mountains, where the 
ridges and peaks are quite commonly the result of the resist- 
ance of some hard layer which is tilted into the mountain 
form (Figs. 219, 225, and 230). Many complexities of moun- 



THE TOPOGRAPHY OF THE LAND. 



405 



tain topography are the result of this etching of folded 
rocks which present differences in hardness. This is seen 
among the Appalachians (Fig. 262), where nearly all of the 
ridges are made of hard strata, and where they form ridges 
because they are more resistant than the surrounding rocks. 

Not merely are there ridges where hard layers exist, but 
peaks (Fig. 220) are often produced where unusually hard 
rocks are found ; and very often, where the general rock 
structure is harder than that of the surrounding regions, 
these places stand up as more elevated areas. Thus the 
Adirondacks, the New England area, etc., are high mainly 
because their rocks are 
prevailingly hard. When, 
by land movements, these 
carved areas are brought 
beneath the sea, their 
irregularities impress 
themselves upon the 
coast line, as for instance 
on the coast of Maine, 
which is a land area 
partly drowned by the sea (Fig. 211). The hard rocks 
which formed hills, now exist as promontories, capes, or 
islands, while the sites of the softer layers are occupied by 
bays or straits. 

From this brief statement, it is seen that the causes for 
topographic irregularities are most complex. They are to 
be found in a combination of internal and external forces. 
The land is in movement, and the forces of denudation are 
at work carving and removing, and often locally build- 
ing. With variations in altitude, position, and kind of 
rock, many complex results may be produced. Above all, 
it must be borne in mind that these changes are now in 




Fig. 262. 

Effect of hard layers (unshaded) in the de- 
nudation of mountains. 



406 PHYSICAL GEOGRAPHY. 

progress ; that the land forms are still changing ; that they 
have been different in the past ; and that the future will 
find them different still. Some forms have reached one 
stage, and some another ; but all are developing along cer- 
tain lines of a more or less definite nature, notwithstanding 
the fact that the conditions are complex, and are even 
undergoing change themselves. Any intelligent study of 
the earth's surface must be made with these facts clearly in 
mind. 



REFERENCE BOOKS. 

There is no easily accessible book in which the relation between scenery 
and geology is more clearly shown, than in Geikie's "Scenery op Scot- 
land." Macmillan & Co., New York. Second edition, 1887. 12mo. $3.50. 
Powell. — Physiographic Features (Natural Geographic Monographs, 

Vol. L, No. 2). American Book Co., New York, 1895. 4to. $0.20. 

(Some suggestive descriptions of the origin of land forms.) 



CHAPTER XXII. 

MAN AND NATURE. 

General Statement. — The relation between man and the 
physical conditions of the earth's surface is most intimate, 
although in his present civilized state they are very much 
less important than in the past. Formerly, even slight bar- 
riers were almost impassable, while now we cross them with 
ease. Less than a half -century ago, the journey from the 
Mississippi to the west coast was of the most dangerous kind, 
while now, in a few days, we pass over the mountains and 
plateaus with ease and comfort. While, with the advance 
of civilization, man is becoming less dependent upon nature, 
at the same time he is increasing his power to control and 
modify the surrounding conditions. So the subject of the 
relation between man and nature naturally divides itself into 
two parts, (1) the influence of nature upon man, which is of 
decreasing importance, and (2) the influence of man upon 
nature, which is all the time increasing. These subjects can 
be treated only very briefly. 

Modifying Influence of Man In many small ways man 

is engaged in the work of modifying the natural conditions 
of his surroundings. He protects himself from the rigorous 
climates, and thus makes his existence possible in zones where 
otherwise he could not dwell. He modifies the forces of 
nature so that they become his servants. The winds, the 
rivers, and even the tides are converted into forces which 
serve him. He confines the river within its banks and pre- 

407 



408 PHYSICAL GEOGRAPHY. 

vents the flood ; and he turns the river waters from their 
course to lead them where he wills. Deserts are transformed 
to fertile gardens ; swamps are made dry ; the sea is excluded 
from the marshy lands of the coast lines; 1 and almost every- 
where we find evidence that man is at work in modifying the 
surface. The earth is pierced with mining shafts and tun- 
nels ; new water connections are made by canals across the 
narrow isthmuses; inland towns are connected with the sea; 
and seashore towns are made into seaports by the construc- 
tion of artificial harbors. 

Notwithstanding the importance of these effects, there is 
no influence of man more potent than that which he exerts 
upon the life of animals and plants. Many species are being 
perpetuated under domestication, and much is being done to- 
ward their modification. New fruits are constantly being pro- 
duced, and in this respect the influence of man is very im- 
portant. Man is doing a great work in distributing animal? 
and plants over regions which are not properly their homes. 
Sometimes the effect is beneficial, but very often it is most 
disastrous. For instance, the rabbit introduced into Australia 
has become a national pest ; and the English sparrow is com- 
pletely overrunning this country. Insect pests and diseases 
are also spread, and these attack not merely man, but also 
the plants and animals. 

However, it is in the destruction of life that the most 
baneful influence of man is noticed. Animals of nearly all 
kinds, particularly some of the largest, are disappearing 
before his advance. Several species have been entirely ex- 
terminated, and some, such as the bison, which was formerly 
so abundant, have been so reduced in numbers that they are 
almost exterminated. By the destruction of birds, the num- 
ber of insects has been increased ; and so both directly and 
1 It is estimated that one-tenth of Holland is land reclaimed from the sea. 



MAN AND NATURE. 



409 



indirectly the influence of man in this direction has been 
harmful. 

Man and the Forest. — Probably the most important single 
influence of man comes from his habit of destroying the 
forest (Fig. 263). In many ways the forest covering is 
important. It protects the soil from being washed away, 




Fig. 263. 

A part of the Adirondack forest. (Copyrighted, 1888, hy S. R. Stoddard, Glens 

Falls, N.Y.) 



and when it is removed, and the soil turned by the plow, 
both weathering and the removal of the loose materials are 
increased. In some places, notably in France, the mountain 
sides, from which the forests have been stripped, have been 
transformed to barren wastes of rock because of the re- 
moval of the soil by the rain. In other places, the soil has 



410 



PHYSICAL GEOGRAPHY. 



been so gullied that it is unfit for cultivation. A part of 
Mississippi lias been transformed to a barren waste of clay, 
the features of which resemble those of the Bad Lands of 
South Dakota (Plate 21). The effect of the absence of 
forests is well illustrated in the arid lands, where the forest 
covering is absent because of natural climatic conditions. 
Here every rain gullies the land ; and on the steeply sloping 
hillsides, the removal of the soil by rain and wind action has 
exposed the bare rock (Figs. 90 and 121). 




Fig. 264. 
Deforesting in the Adirondacks. 

The forest serves to prevent excessive river floods ; for it 
protects the snows from rapid melting, and prevents the rain 
from readily passing away in the streams. The mat of leaves 
and moss, the forest Utter, serves as a great sponge which 
holds the water. This is important in many ways : for it 
makes the stream less liable to violent floods ; it furnishes 
a constant and rather steady supply, both to springs and 
streams; and it furnishes moisture to the air. With the 
removal of the forest covering, the rain and the melting snow 






MAN AND NATURE. 



411 



pass rapidly into the rivers, and thence to the sea (Fig. 264). 
At times, exceptional floods are produced ; and then, when 
these have passed away, the river rapidly loses in size, until 
it may perhaps become nearly, if not quite dry (Fig. 124). 
The greater part of the water passes through the river in a 
few days. Every person of maturity who has dwelt by the 
side of a stream heading in a region once forested, but now 
bared of its tree covering, will bear testimony to the fact 




Fig. 265. 
Bare rock exposed to weathering by removal of the forest, Mt. Desert, Me. 



that streams which were formerly moderate, clear, and per- 
manent, are now transformed to trickling streams, which at 
times become raging torrents, clouded with sediment. 

This influence of man is very disastrous. It not merely 
causes the removal of soil from the mountains (Fig. 265), 
but distributes this over the lowlands ; and in some places, 
farms have been rendered uninhabitable by the deposit of 
sediment during times of flood. Besides this, the floods 
themselves are very destructive both to life and property ; 



412 PHYSICAL GEOGRAPHY. 

and, with the removal of the forest covering, they are becom- 
ing ever more destructive. Mills cannot count upon the 
same steady water supply that they formerly had ; springs 
quickly become dry; and there is some reason for believing 
that the removal of the forest also affects the climate. This 
latter point has been suspected ; but it has never been 
proven that the forest makes the rainfall more uniform or 
greater in quantity. The reasons for suspecting this forest 
influence are (1), that the damp winds, when coming in con- 
tact with the cool forests, are made to give up their moisture 
more readily than elsewhere ; and (2) that by holding the 
water in the litter beneath the trees, a greater opportunity 
for evaporation is furnished than when the forest is removed. 
It is held that, as a result of this, the air is rendered moist 
and is more liable to give up its moisture. 

These influences are so important, that one of the needs of 
the present, is greater care, intelligence, and patriotism in the 
relation of man to the forest. The conditions need to be 
carefully studied, destruction ought to be checked so far as 
possible, and the damage of past destruction should be 
repaired in every possible case. The state and national 
governments are in some cases engaged in this work ; but it 
is possible for nearly every one to do something toward it. 
Unless something is done, the heritage of the land which we 
have received, will not be transmitted to our descendants in 
so good a condition as it is our duty to leave it. 

Influence of Nature upon Man It is quite impossible at 

present to estimate the effect of nature upon man ; for in 
most respects we have risen above its immediate and most 
important modifying influences. Without serious difficulty, 
we cross mountains and continents, rivers, lakes, and even 
oceans ; and in a few weeks we may pass around the entire 
world. Every generation sees an increase in the independ- 



MAN AND NATURE. 413 

ence between man and nature, and the completeness of the 
conquest of the latter. 

This has not always been so, and many of man's most 
marked characteristics have had their origin in, or have been 
impressed upon him by his environment. Even now we find 
a marked difference between the miner, the ranchman, and 
the farmer ; and, except in the most general way, the effect 
of climate upon man's condition cannot even be estimated. 
Both extremes of heat and cold introduce habits of mind and 
body quite the reverse from the lively mental and physical 
activity of the inhabitants of the temperate zone. The in- 
habitant of the Arctic loses vitality because of the unequal 
struggle ; and where no severe struggle for existence is 
necessary, the enervating influence of the tropical sun also 
decreases vitality. Under the bracing air of the temperate 
latitudes, and with the necessity for preparation for the win- 
ter, man's physical and mental powers have been improved; 
and this is probably the most potent reason for the very 
striking fact, that the most important development of the 
race has taken place in these regions ; and why to-day, nearly 
every nation of marked inrportance is situated within the 
temperate belt, and mostly near the arctic limit of it. 

If we glance back to the time when man was less inde- 
pendent of nature, when his railway trains were not present 
to transport him across river and mountain, nor his steam- 
ships ready to bear him across oceans, we find a very close 
relation between the life of men and nations and the sur- 
rounding physical conditions. When a people migrated to 
a new land, they often found conditions favorable to a rapid 
development ; and if they were sufficiently enclosed and 
isolated for protection from invasion, they often developed to 
a high state. However, in time the very isolation caused 
degeneration ; and we see this illustrated in the history of 



414 PHYSICAL GEOGRAPHY. 

such people as the Chinese and the Egyptians. Because of 
the climatic conditions under which they lived, some' of the 
early people became nomadic, others developed into agri- 
cultural nations, and still others into seafaring races. 

With the growth of commerce, the most rapid progress 
occurred in the nations most favorably situated for its devel- 
opment. Thus Italy, nearly isolated from other countries, 
became an important center of commerce. Fresh blood and 
new ideas were constantly introduced, and gradually the 
power of the nation increased and extended, until decay came 
mainly as a result of the rapid development, and the nation 
crumbled because of its very success. In ancient Greece we 
find another instance of the influence of surrounding condi- 
tions. The very rugged topography favored independence : 
for even in small areas, different states could exist independ- 
ently ; but because of their very smallness, they were com- 
pelled to unite against common foes. 

The Mediterranean was the seat of early development; 
and this was made possible by reason of the short distances 
between countries, the possibility of navigation in the enclosed 
sea, and the interchange of materials and ideas. Here the 
people learned lessons in navigation which made the explora- 
tion of the Atlantic less hazardous ; and then, by land and by 
sea, the peoples from the shores of the Mediterranean taught 
lessons to the more northern races, which, when well learned, 
made their ultimate success possible ; and then the students 
themselves became leaders. The shores of the Mediterranean 
were the great training schools, in which were learned most 
of the fundamental ideas upon which the progress of the 
human race has depended ; and even now its influence is 
felt most markedly in all the nations of the world. 

Perhaps there are no better illustrations of the influence 
of surroundings upon the development of nations, than those 



MAN AND NATURE. 415 

furnished by Scandinavia and England. The roving North- 
men, born on a rocky coast, which was deeply indented with 
fjords, made the sea a second home. Instead of farming, they 
fished ; instead of remaining to develop their inhospitable 
coast, they roved the seas and invaded their neighbors. 
With that hardiness born of the sea, they roamed not only 
along the European shore, but sought and discovered new 
lands in the west. They not only learned much themselves, 
but taught much to others ; and the lessons that they 
learned and imparted were mainly due to their neighbor- 
hood to the sea. 

In England, we find a most remarkable illustration of the 
influence of environment. The climate gave vigor to the 
people ; and the mixture of races, that had come in earlier 
days, made a nation of men with great mental and physical 
power. The mineral wealth did much to make the subse- 
quent development possible, for it became sought after by 
many nations. Because of the insular condition, this store 
of wealth was protected without great difficulty ; and yet the 
islands were readily visited for purposes of friendly com- 
merce, and the stores of wealth were distributed over the 
world to the profit of the people of the islands. A com- 
merce was readily developed ; and largely upon the basis of 
this, England became what she is to-day, — ■ the great naval 
power of the world, and the possessor of colonies in every 
part of the earth. It never can be told how important an 
event it was in the development of nations, when, in some 
prehistoric time, the sea first passed through the English 
Channel, and separated the British Isles from the mainland. 
With land connection, the history of Europe and the world 
might have been quite different. 

When we look at the maps of Europe and America, two 
differences of a most striking nature attract our attention. 



416 PHYSICAL GEOGRAPHY. 

The one is the extreme irregularity of the European coast 
line, the other the great number of nations in that land. 
The latter fact depends upon several causes. The very 
irregularity of the coast, and the great diversity of the 
topography, have made possible the development of distinct 
nations. As the race was progressing, mountain barriers, 
and even rivers, served as boundary lines between separate 
tribes ; and some of these are preserved to this day. We 
find Switzerland completely enclosed between other nations, 
because no ancient tribes could drive these people from their 
mountain fortress. To fully appreciate the importance of 
these influences, one needs but examine a physical map of 
Europe, and notice how the mountains and the seas almost 
universally serve as boundaries, and how upon every penin- 
sula, there is one, or more, independent nation. This is not 
so in America, partly because the conditions are not so diverse, 
but chiefly because the settlement of America was made by 
races which had already developed. 

With the development of knowledge and power, there came 
an era of exploration, in the course of which America was 
discovered. Even this discovery depended upon peculiar 
physical conditions ; for had Columbus undertaken to make 
his voyage either to the north or south of the trade-wind 
belt, the chances are that he would never have succeeded. 
With the favorable trades furnishing fair winds, the journey 
was a relatively easy one. 

The explorers and settlers found the American land occu- 
pied by nomadic races, whose power of resistance to invasion 
was not equal to the skill of the invaders ; and with the 
discovery of America there began a new era. Navigation 
increased ; and Spain, who had learned her lesson from Italy, 
and who was important in maritime affairs because of her 
extensive coast line, became a powerful nation. As in Italy, 



MAN AND NATURE. 417 

success caused almost utter collapse, and Spain lost more 
than she gained. 

In America, the invigorating climate, the necessity of work, 
and the great possibilities, developed a race which has become 
renowned for its vigor and energy. At first the Appalachian 
barrier, with its almost impassable forests, prevented entrance 
to the central regions. Therefore, of necessity, settlements 
were made close by the coast ; and it is said that in 1700, one 
could go by stage from Portland, Maine, to Virginia, spend- 
ing every night in a good-sized village, while to the west there 
was an impassable wilderness traversable only on foot, along 
the Indian trails. The large waterways leading into the 
interior were guarded, the Mississippi and the St. Lawrence 
by other nations, and the Mohawk by a powerful Indian 
population. 

This forest barrier caused a concentration of population, 
upon which much of the success of our later development 
has dependedo It determined the location of most of the 
great centers of population ; and it protected the English 
until their strength had sufficiently increased to admit of 
pushing into the western region, and displacing the unfor- 
tunate savage occupants. The success of the Revolution also 
in great measure depended upon the concentration of pop- 
ulation thus induced. Had the people been less connected, 
they could not have cooperated so well as they did. 

When, finally, a definite roadway was established across 
the Appalachians, which was first done over Cumberland Gap 
in Tennessee, the most difficult step in the western progress 
was taken. The great treeless prairies were then reached, 
and upon these agriculture was easily pursued, while further 
progress was not difficult ; and hence the Mississippi valley 
became speedily developed. When once the way was found, 
other openings were soon made across the forest barrier. 

2 E 



418 PHYSICAL GEOGRAPHY. 

Then came the discovery of the wonderful mineral wealth 
of the west ; and the eagerness to obtain some of these stores 
from the bosom of the earth, caused an almost magical de- 
velopment of this great realm. Cities sprang up among the 
mountains, farms were developed on the desert, railroads 
crossed the mountain chains, states grew out of hitherto un- 
settled territories ; and, in a quarter of a century, a great 
region was transformed from an unknown waste, inhabited 
only by savages, to the most remarkable mineral-producing 
region of the world. Such progress as this could be made 
only after man had so far developed as to be able to defy 
and overcome the most formidable of obstacles. 

In this country, the influence of topography upon man is 
seen in many small ways. In New England, particularly in 
central Massachusetts, the old interior towns were on the hills, 
which were fortresses where the people were, in a measure, 
safe from Indian attack ; and even now we find many of 
these hilltop villages, which at present are scarcely more 
than relics of a past stage in development. With the devel- 
opment of the industries, manufacturing determined the posi- 
tion of the more important interior towns ; and these were 
naturally placed in the valleys which afforded a good supply 
of water power. 

Hilly New England became a manufacturing region ; the 
states of the level and fertile prairie formed an agricultural 
district ; the drier plains and plateaus of the west became the 
seat of the cattle industry ; and the mountainous region of 
the far west developed into a mining territory. Many of 
the larger cities were situated on the seacoast, because here 
communication and commerce with other countries were pos- 
sible. Even the sites of these large cities were determined 
by the form of the coast line ; and everywhere that we may 
go in the world, we find an almost universal relation between 



MAN AND NATURE. 419 

man's condition and his surroundings. The delta lands are 
farming districts, the semi-arid plains and plateaus are 
devoted to cattle raising, etc.; but while man is largely a 
creature of his environment, he is much less so now than 
ever before ; and, little by little, he is rising above the 
necessity of direct dependence upon the surrounding physical 
conditions. Formerly he was guided by nature, but now, in 
many respects, he governs and guides nature to suit his needs. 



REFERENCE BOOKS. 

Shaler. — Nature and Man in America. Scribner, New York, 1891. 

12mo. $1.50. 
Guyot. — The Earth and Man. [Translated by Felton.J Scribner, 

New York. Kevised edition, 1893. 12mo. $1.75. 
Marsh. — The Earth as Modified by Human Action. Scribner, New York, 

1885. 8vo. $3.50. 



CHAPTER XXIII. 

ECONOMIC PRODUCTS OF THE EARTH. 

Soil. — The crust of the earth furnishes to man most of 
the material which he needs for life and comfort. The 
rocks crumble to form soil, and upon this exist the plants 
which furnish us directly or indirectly with most of our food 
supply. In this the trees grow, and all of the animals of 
the land depend upon the plant life which exists by virtue 
of this soil covering. This is by far the most important 
mineral product of the earth, for upon it depends our exist- 
ence as inhabitants of the land. 

Building Stones. — Within the earth, as a part of the 
crust, there are many substances which man finds it possible 
and profitable to remove for his own use. For instance, 
there are the building stones, of which we have many kinds. 
The great masses of molten rock, which have been intruded 
into the earth's crust from below, and then cooled, and finally 
reached by denudation, furnish us with great quantities of 
granite, which is such excellent building stone, both with 
regard to durability and appearance. Sometimes other 
forms of igneous rocks are employed for building pur- 
poses ; and among these we find great variety both in 
color and texture. 

Granite is imitated among the metamorphic rocks, where 
as a result of the process of alteration, a structure closely 
resembling that of granite is sometimes introduced into the 
gneissic layers. Indeed, many gneisses are sold as granites, 

420 



ECONOMIC PRODUCTS OF THE EARTH. 421 

and their resemblance is often so close that one can tell the 
difference only by a slight banding which characterizes 
gneisses, but is not usually present in granites. 

There are other metamorphic building stones, chiefly slate 
and marble. Slate represents a clay rock formed as a deposit 
in water, and then subjected to heat and pressure, so that its 
peculiar cleavage is introduced. Marble is the metamor- 
phosed product of limestone, in which the carbonate of lime 
has in some cases been transformed to crystals of calcite, 
causing the white sugary marble, such as that found in Ver- 
mont. In other cases no crystals are produced, but a re- 
markable and often very beautiful banding is introduced. 
The causes for this metamorphism are usually a combination 
of heat, pressure, and motion during the folding of the rocks. 

The sedimentary rocks themselves also furnish us with 
much building stone, chiefly in the form of sandstone and 
limestone. Among these there is great variety, both as re- 
gards texture and color ; and this class of building stone 
is extremely common. Indeed, these rocks are so abundant 
that only the best can be extensively used; and in many 
places a stone is quarried for home use, but is never trans- 
ported far beyond the quarry. Few stones, and these mainly 
ornamental, will pay for transportation to great distances, 
for there is an abundance of stone for ordinary purposes, 
and nearly every place has its quarry. 

From the unconsolidated clays and sands, we obtain much 
material for building purposes. The sand for plaster, the 
clay materials for some cements, and the clay for bricks, are 
among the most important of building materials, and their 
sources are varied. Some are decayed rocks, others ocean 
deposits, others have been formed by rivers or lakes, and 
many, particularly in northern United States, have been 
brought to their present position by glacial action. 



422 PHYSICAL GEOGRAPHY. 

Economic Deposits of Sedimentary Origin. — Aside from 
the sedimentary building stones, and some of the ores, the 
crust of the earth contains numerous valuable deposits 
formed in water. Some of the sandy rocks are sufficiently 
rough to be used for grinding purposes ; and the tiny shells 
of silica, which are left in fresh-water swamps and ponds by 
certain low forms of animals and plants (Infusoria and 
Diatoms), furnish a white polishing powder. 

When lakes have their outlets cut off, and evaporation 
exceeds the supply of water, they gradually become salt; 
and finally they may become so concentrated that some is 
deposited in the bottom of the lake, and then there is formed 
a layer of rock salt. These layers may be buried beneath 
other strata, and at some later time be discovered as a salt 
mine. In the Great Basin there are many beds of this kind, 
now exposed at the surface, where in some recent times a 
salt lake has completely dried up ; and the ranchmen visit 
these beds with wagons, and shovel up from the surface all 
the salt that they need. 

At times there are other materials deposited with this 
precipitated salt. In this way such substances as bromides, 
borax, natural soda, and even gypsum, which is used as a 
basis for plaster of paris, are deposited in layers. 

Left to itself, the soil furnishes the plants as much food as 
they require ; but when man interferes and tries to draw 
more than this from the soil, in the course of time he ex- 
hausts much of the supply of plant food, and it is then 
necessary either to abandon the land until it can recover, 
or else to artificially supply the needed substances. For 
this reason fertilizers of one kind or another are added. 
Sometimes the fertilizer is only a limestone, or it may be a 
marly clay in which there are many fossil shells, or it may 
be one of the natural phosphates. Phosphatic materials are 



ECONOMIC PRODUCTS OF THE EARTH. 423 

among the substances needed by plants, and phosphate is 
present in the bones of many mammals. In some places, as 
for instance in South Carolina, near Charleston, and in many 
parts of Florida, there are beds of a phosphatic rock which 
owes its peculiar character to the presence of large numbers 
of bone fragments. These are great mammalian burial 
grounds, and man is now drawing upon them with profit. 

Miscellaneous Substances. — There are many other products 
of the earth, which though valuable, are of minor importance. 
Springs containing mineral matter in solution often have 
medicinal properties, and this ensures a wide sale for these 
mineral waters. Artesian wells (page 229) are of no little 
importance. Sulphur is often found near volcanoes ; graph- 
ite, which occurs in metamorphic rocks, furnishes the black 
lead for our pencils ; mica, asbestos, etc., are also found in 
the metamorphic rocks ; valuable mineral paints are usually 
colored earths due to rock decay ; and to these many other 
minor products might be added. 

Coal. — Seams or beds of coal are often found between 
layers of sandstone and limestone. These enclosing rocks 
bear evidence of having been formed in water, and the fos- 
sils which they contain often prove that they were deposited 
in salt water. Yet the coal is composed of the remains of 
land plants, and even tree trunks are sometimes found pre- 
served in the beds. In some cases these fossil tree trunks 
stand upright, with their roots in the clay beneath, showing 
that the coal bed is near the place where the plants grew. 
It is further evident that they were then covered by the 
sea, and that in this the marine sediment was deposited. 
Often there are several beds one above another, each prov- 
ing some such change as this. 

Much about the origin of coal cannot be considered to be 
finally settled ; and there are many theories for its origin. 



424 PHYSICAL GEOGRAPHY. 

Since we cannot enter into a discussion of these, it will be 
necessary to confine ourselves to a statement of what seems 
to the author to be the most probable explanation. Without 
doubt, different coal beds have had a very different history, 
Some represent the drifted fragments of wood that have 
been deposited in an ancient bay or estuary, and then 
buried beneath marine deposits. Thus if the Mississippi 
delta should be consolidated into rock and be elevated, there 
would be coal seams formed where rafts of logs have been 
stranded. 

There also seems to be no doubt that some coal beds are 
nothing more than swamps which were formed either on 
shores of lakes, or as the last stage in their disappearance, — 
in a measure being like peat bogs consolidated to mineral 
fuel. In the southern part of Florida there are a great num- 
ber of swamps, and swampy lakes, in which there is a vegeta- 
ble accumulation several feet in depth. This muck is made 
almost entirely of plant remains with practically no clay 
impurities. If this low, swampy land were to be lowered 
beneath the sea, these beds of vegetable matter would be 
covered with sediment, and a coal bed would be begun. 
Later the same conditions might be repeated and another 
bed be formed, etc. 

Even at present, some trees (the mangrove, Fig. 205) grow 
in salt water; and in the early geological ages many others 
probably had this habit, for the land vegetation of these early 
times was evolved from marine plants. At this time there 
were probably great salt-water swamps, in which many of the 
coal beds were formed. Very likely each of these theories 
accounts for some of the beds. 

The coal is a mineralized form of vegetation, produced by 
a slow change, in the course of which many of the volatile 
gases have been driven off. There is every gradation from 



ECONOMIC PRODUCTS OF THE EARTH. 425 

wood to peat, from this to lignite or brown coal, then to 
bituminous, next to anthracite, and finally even to graphite. 
This does not require great heat, but slow, steady change. 
The ash of the coal is an impurity, often bits of clay and sand 
that were deposited with the coal. 

It was once supposed that coal was formed only at one 
period in the history of the earth, and this was given the 
name Carboniferous; but with the exploration of the Cor- 
dilleras, this has been shown to be a wrong idea. Workable 
beds of coal were not formed before the Carboniferous time, 
because in those early ages there was not enough land vege- 
tation; but ever since this time, coal has been formed where- 
ever the conditions have been favorable. In the west there 
are vast quantities of Cretaceous and Tertiary coal. Indeed, 
in such places as the swamps of Florida, the Dismal Swamp, 
and the peat bogs of the north, it is quite probable that we 
are even now witnessing the first stages in coal accumu- 
lation. 

Natural Gas and Petroleum. — In some places, wells drilled 
into the sedimentary rocks reach layers containing either a 
natural illuminating gas, or petroleum. These products are 
very useful, the gas for fuel and light near the wells, the oil 
for the basis of kerosene, and numerous other products. 
These substances occur rather irregularly ; and wells upon 
neighboring farms may in the one case find oil, while this is 
not discovered in the neighboring well. However, certain 
layers are liable to be oil bearing, while others are never 
known to contain oil or gas. After awhile both the oil 
and gas wells gradually decrease in volume, and must finally 
be abandoned. Therefore the supply is not constantly fur- 
nished at as rapid a rate as the drain. 

These substances are the product of a slow natural distil- 
lation of the organic remains of the rocks; and they quite 



426 PHYSICAL GEOGRAPHY. 

closely resemble substances which we produce artificially. 
The oil is not markedly different from that produced from 
fish refuse ; and the gas resembles the illuminating gas 
caused by distilling coal. The change is a slow one, and 
in the course of time, enough accumulates in a certain 
layer to make a gas or oil deposit. In some cases this 
accumulation is in the same layer in which the distillation 
took place ,• in others, the substances have migrated into a 
neighboring layer. Like water, they are able to slowly 
seep through the rocks ; and in their passage, they may 
come into a coarse sandy rock, and be imprisoned there 
by an overlying clay layer which is too impervious for easy 
passage. In these cases there is a resemblance to the con- 
ditions favoring artesian wells. This is the common case 
in the Pennsylvania wells ; but in Indiana these substances 
occur in a limestone. 

Ore Deposits. — Some of the metals which occur in the 
earth possess qualities which make them useful to man; and, 
as we know, great effort is made to obtain them. Iron, gold, 
silver, copper, etc., serve us in many ways. In the earth 
they generally occur in association with other elements, in 
the form of minerals ; and when mined, these have first to be 
separated from their companion minerals, with which they 
are mechanically mixed ; and then it is usually necessary to 
separate the metal from the elements with which it is chemi- 
cally combined. Therefore in obtaining these substances 
from the earth, many complex and often very costly methods 
are employed. In order that this may be profitable, the ores 
of the metals must occur in a somewhat concentrated condi- 
tion, and they must be in a place from which they may be 
obtained without too great expense. Thus a copper mine 
that would pay in New England or New York, might not be 
profitable if situated among some of the nearly inaccessible 



ECONOMIC PBODUCTS OF THE EARTH 427 

mountains of the west. Where the deposit is very rich, it is 
often profitable to tunnel into the earth to a depth of several 
thousand feet. 

Ores occur in the rocks of the crust under many different 
conditions. Sometimes the ore is a native metal, as is the 
case with most of the gold which is mined ; but more 
commonly it is a simple compound of a metal. It would 
be quite impossible to state in a few words the various 
ways in which the ores occur, and only one or two of the 
most common kinds can be described, and these only in a 
general way. 

Some of the ores have been deposited in beds, by a process 
of replacement. That is, some mineral or rock, such as 
quartz or limestone, has had its place taken by the ore, — 
this being deposited bit by bit, while the water which car- 
ried the solution took away an equal amount of the original 
mineral. This resembles the replacement of wood tissue by 
silica — a process known as petrefaction. Some ore deposits 
represent the mere gathering together of substances into 
bunches, known as concretions, the cause for the accumu- 
lation being still unsolved. 

Much more commonly, ores are deposited in some cavity in 
the earth ; and the most common of these is the fissure which 
accompanies faulting. In this break in the strata, which 
often extends to great depths, ore is deposited from solution 
in water. This underground water is often highly heated, 
and contains in solution alkaline or acidic substances which 
give to it great power of dissolving and altering min- 
erals. By complex chemical reactions, which are not well 
understood, these ores are deposited in veins, usually in 
bands, and commonly associated with other minerals which 
are not of value. Even ores of gold or silver are frequently 
deposited in this way. 



428 PHYSICAL GEOGRAPHY. 

Another important way in which ores occur, is in surface 
deposits of sedimentary origin. For instance, when a gold- 
bearing rock decays, the nearly indestructible gold resists 
weathering ; and being a heavy substance, as it is being 
washed down toward the sea, it tends to accumulate on the 
stream bottom, forming what is known as stream or placer 
gold deposits. This is the condition in which a great deal 
of the gold of the world has been found ; and this precious 
metal occurs in such deposits in the west, in Siberia, Aus- 
tralia, and many other places. Both tin and platinum are 
also found in a similar condition. 

Distribution of Ore Deposits. — The valuable ore deposits 
which are found in fissures, are not present in all parts of the 
crust ; but for the most part they are confined to mountainous 
regions. The Cordilleran region of the west is a most strik- 
ing illustration of this ; for these mountains form the most 
remarkable mineral district of the world. While this dis- 
trict produces only a few of the metals, it is not because the 
others (such as iron) are absent, but because in that region 
the conditions are too unfavorable for the extraction and 
marketing of those which are not very valuable. 

The reasons for the great importance of the Cordilleras in 
the production of metals, are mainly two. In the first place, 
among these mountains there are many faults, and other 
cavities, in which ore may be deposited. There are also 
numerous volcanic rocks of recent date, a point of consider- 
able importance. The heat from these lava intrusions fur- 
nishes to the underground water a temperature sufficiently 
high for important action. Probably even at present some 
of the hot springs of that region receive their heat from 
buried lava intrusions ; and probably also, mineral deposits 
are being made in their tubes at a considerable distance from 
the surface. A second reason why the presence of igneous 



ECONOMIC PRODUCTS OF THE EARTH. 429 

rocks aids in ore formation, is that there is a larger percent- 
age of metals in these than in others. Therefore water 
which is percolating through and altering them, finds a 
greater supply of metals for solution than would be the 
case if passing through most sedimentary rocks. 

Mineral Wealth of the United States. — Mainly because of 
the Cordilleras, the United States is the great mineral 
country of the world. Of the following metals it produces 
more than any other country : gold, silver, iron, and copper, 
which are the most important of metals ; and in the pro- 
duction of lead, zinc, and mercury, it holds second rank. 
Its output of coal is greater than that of any other nation 
excepting Great Britain, while no other country supplies so 
much petroleum and natural gas. In some of the minor 
substances it also holds a high rank. Indeed, we produce 
nearly every important mineral substance found in the 
earth's crust ; and usually our production is very great. 

The importance of the mineral industry of this country, is 
shown by the fact that in 1892 the mineral production was 
valued at nearly $700,000,000, of which about $300,000,000 
came from the metals, — mainly iron, silver, gold, copper, 
lead, zinc, and mercury. For the most part this represents 
the crude product ; and in the utilization of this in manu- 
facturing, there are industries also worth many hundred 
millions of dollars : so that, directly and indirectly, the min- 
eral industry of the country is one of the most important. 

The few facts that follow, will serve to furnish an idea of 
the distribution of this product. According to the census, 
the leading mineral state is Pennsylvania, which produces 
more coal, petroleum, gas, and stone, than any other state. 
In 1889 the value of its product was $150,000,000. Second 
in rank is Michigan, which produces most iron and salt, and 
is the second in the production of copper. Then comes 



430 PHYSICAL GEOGRAPHY. 

Colorado, which leads in the production of silver and lead, 
and is second in the production of gold ; and Montana fol- 
lows, leading in the production of copper, and second in the 
output of silver. The east excels in the production of non- 
metallic substances, and the west in metals. 

This astonishing mineral wealth has, in no small degree, 
been responsible for our development as a nation ; and there 
are still great undiscovered stores. There seems to be almost 
no limit to the possibilities in this direction, and our Alaskan 
territory promises to add to this wealth. Nature has been 
most prodigal in lavishing her favors upon this country, for 
she has given us nearly all that man could request : great 
variety of climatic conditions, an almost infinite variety of 
topography, a soil wonderfully rich over a great area, a forest 
covering from which we have been able to draw heavily for 
over a century, water power for the mills, harbors for the 
commerce, mineral deposits of marvelous wealth, — these 
are things which mark our country as one of great possi- 
bilities, and which have made possible our present prosperity, 
and upon which we may predict so much for the future. 



REFERENCE BOOKS. 

Kemp. — The Ore Deposits of the United States. Scientific Publishing 

Co., New York, 1893. 8vo. $4.00. 
Phillips. — Ore Deposits. Macmillan & Co., New York, 1884. 8vo. $7.50. 
Tarr. — Economic Geology of the United States. Macmillan & Co., 

New York. Second edition (revised), 1895. 8vo. $3.50. 



APPENDIX I. 

METEOROLOGICAL INSTRUMENTS, APPARATUS, AND 
METHODS. 

By instruments we are able to measure the temperature, pressure, wind 
force and direction, rate of evaporation, percentage of moisture in the 
air, amount or percentage of sunshine, rainfall, and other weather phe- 
nomena. In order to understand these instruments, it is necessary to 
handle them just as the meteorological observer does. Mere descrip- 
tion can serve only to explain the principle upon which they depend. 

Thermometric Records. — In measuring the temperature, use is made of 
the principle that certain substances expand when heated and contract 
when cooled. Ordinarily it is more convenient to employ a liquid, and 
that best adapted to this purpose is mercury. However, where tem- 
peratures below the freezing-point of mercury are liable to be experi- 
enced, alcohol is used. 

The thermometer is graduated into degrees according to some scale, and 
different scales are employed, the most common in use being the Fahr- 
enheit, which is adopted in nearly all English-speaking countries, and 
is used in this book. The two points of importance in the Fahrenheit 
scale are the freezing-point, which is placed at 32°, and the boiling- 
point, which is placed at 212°. In the Centigrade scale, the principle is 
the same ; but in this case the degrees are larger, the freezing-point being 
placed at 0°, and the boiling-point at 100°. Therefore, in converting the 
Fahrenheit to the Centigrade scale, 1° of Centigrade is equal to 1.8° of 
Fahrenheit, and to this must be added 32°. All are familiar with ther- 
mometers, and the principle upon which they depend is easily under- 
stood. 

Much care is needed in the construction of a good and accurate ther- 
mometer, and there are some cheap and very inaccurate instruments. 
This is one reason why the observations of temperature made by different 

431 



432 PHYSICAL GEOGRAPHY. 

people may vary so widely, even though made in almost the same location. 
Another very important reason for this difference is the fact that the 
thermometer is not always wisely placed. In order to obtain a true 
measure of the temperature of the air, it is necessary that neither the 
sun, nor any warm body on the earth, shall influence the air whose tem- 
perature is to be measured. At meteorological stations, the thermometers 
are placed in a thermometer shelter, which consists of a frame, open so 
that the air may pass through it, and yet sufficiently closed to prevent 
the sun's rays from striking upon the thermometer. This is raised about 
10 feet from the surface, and is placed away from buildings. 

Of late, metallic thermometers have come into use ; and these depend 
upon the effect of heat and cold on metal strips or springs enclosed 
within a clock-like case. They are not so accurate as the well-made 
mercurial thermometers, and their chief value is in obtaining a continu- 
ous record. The self-recording thermometers, or thermographs, are mostly 
of this class. As the metal expands or contracts, it causes an index hand 
to move back and forth over a dial, and upon this index, a pen or pencil 
may be fixed in such a manner as to press against a sheet of paper. As 
the temperature rises and falls, the needle is made to move backward 
and forward, and therefore the pen is also moved over the paper. For 
the purpose of obtaining a record of the time at which these changes 
occur, the paper itself is also made to move by means of a clock-work 
attachment ; and therefore a record of all the temperature changes 
throughout the day, may be automatically registered. 

It is often found desirable to have a record of the highest and lowest 
temperatures of the day made by a mercurial thermometer. For this 
purpose the maximum and minimum thermometers are used, which, by a 
special contrivance, record the very highest and lowest temperatures of 
the day, but do not give any record of the time at which these occurred. 
The thermometer itself gives us a record of the air temperature, which 
is very different from the energy which comes from the sun. If the 
bulb of a thermometer be blackened by black paint or lampblack, and 
the instrument be placed in the direct rays of the sun, it is found that 
the temperature rises very much higher than in the case of a thermom- 
eter in the shade, or even of a natural thermometer exposed to the sun's 
rays. Such an instrument is known as the black-bulb thermometer. 

Barometric Records. — The air has weight, and at the sea level this 
weight, or air pressure, averages approximately 15 pounds on every 
square inch. The air pressure at any given place is liable to many varia- 



APPENDIX I. 438 

tions, and it is the purpose of the barometer to detect these changes. The 
principle of the barometer is that a column of air will exactly counter- 
balance a column of equal weight of any liquid. Thus water in a 
vacuum will be made to rise to a height of about 32 feet. In other 
words, it counterbalances the pressure of the air, and the pump is 
based upon this principle, water being forced into the partial vacuum 
caused by pumping. We could use a column of water for a barometer 
just as well as mercury, which is ordinarily used ; but such a barometer, 
since it would need to be at least 35 feet in height, would be most 
unwieldy. The mercurial barometer consists of a tube of glass, sealed 
at one end and partly filled with mercury. Above the column of mer- 
cury is a pi'actical vacuum, and the. lower part of the tube is immersed 
in a cistern of mercury. As the air pressure varies, the mercury is caused 
to rise in the tube, or to descend from it into the cistern ; and when the 
air is heavy, we speak of a high barometer; when it is relatively light, of 
a low barometer. In meteorology, these terms have come to be synony- 
mous with high pressure and low pressure. 

The tube of the instrument is graduated in inches, and at the sea 
level the average height of the mercury in the barometer is about 30 
inches. The method of reading the barometer, and the use of the 
vernier scale, can be understood only by handling an instrument. 

Several forms of barograph are employed to convert the record of the 
change in pressure into graphic, continuous records. The rising and 
falling column of mercury may be automatically photographed, or the 
rise and fall of the column may be recorded by electricity; but most 
commonly some form of aneroid barometer is employed. The aneroid 
depends upon the effect of air pressure upon a metallic diaphragm ; and 
as the index hand moves one way or the other, it carries a pen, which 
marks the changes upon a sheet of paper revolving on a cylinder, just 
as in the case of the self-recording thermometer. 

Measurement of Wind Direction and Force. — To-day the direction of 
the wind is measured in very nearly the same manner that it has been 
for centuries. The wind vane is a familiar feature. The force of the 
wind, or its velocity, may be roughly estimated by any observer. A state- 
ment that the wind velocity is 40 miles an hour, means that in one hour 
the wind travels that distance. For accurately measuring this velocity, 
an instrument known as the anemometer is used. It consists of four cups, 
fixed Upon a cylinder, which are revolved by the wind at rates depend- 
ing upon its velocity. The air enters these cups and whirls them about, 
2f 



434 PHYSICAL GEOGBAPHY. 

very much as water enters a turbine wheel and causes it to revolve. By 
means of a series of wheels, each revolution of the anemometer is re- 
corded, and this may be transmitted by electricity to some place where 
an automatic record is kept in miles per hour. 

Measurement of Evaporation. — The measurement of evaporation is 
made in inches of water evaporated from a surface exposed to the air. 
Almost any dish can be used, and the scale of inches be marked upon 
it ; or the measurement may be made with a graduated rule. Since the 
rate of evaporation varies with the temperature, it is best to attempt to 
imitate natural conditions as nearly as possible, though this is not ordi- 
narily done. The best way is to place the evaporating pan in a quiet 
body of water, allowing it to float on the surface. There are various 
contrivances for obtaining a continuous record. 

Measurement of Moisture in the Air. — The measure of the relative 
humidity is often obtained by the hair hygrometer, which is a bundle of 
human hair from which the oil has been extracted. As the amount of 
moisture in the air increases, the hair absorbs more and more, and as it 
does so, expands; and, since one end is fixed while the other moves 
freely, this expansion may be made to record itself against a graduated 
glass scale. 

The best method is that of the use of the sling psychrometer. This instru- 
ment consists of two thermometers fixed side by side upon a board. One 
is an ordinary thermometer, the other has a piece of wet muslin placed 
around its bulb. The instrument is whirled in the air, and the water 
evaporates from the wet muslin, the rate of evaporation varying with the 
humidity of the air. If the air is very dry, evaporation takes place 
rapidly ; if damp, it proceeds with slowness. Since evaporation produces 
cold, the temperature of the wet bulb thermometer descends lower than 
that of the ordinary thermometer. By reading these two records of tem- 
perature, the relative humidity of the air is readily determined by means 
of a series of tables which are constructed for and furnished by the 
Weather Bureau at Washington. The relative humidity is expressed in 
per cents between 0, which is perfectly dry air (a condition which never 
occurs), and 100, which is saturated air. From this measure the dew- 
point may also be determined. 

Study of Clouds and Sunshine. — Various instruments are used to 
obtain a record of the amount of sunshine, and these may be found 
described in the books referred to at the end of this Appendix. Much 
work of a scientific nature is also being done in the study of clouds, in- 



APPENDIX I. 435 

eluding the measurement of height, the photographing of cloud forms, 
etc. We cannot devote space to a description of these. 

Measurement of Rainfall. — By the rain gauge, rainfall is measured in 
inches, an inch of rainfall being an actual inch of water which has fallen 
upon the surface. This is a cylinder having a broad, funnel-shaped top, 
with the outlet to the funnel extending into an inner cylinder. The 
water falls upon the surface of this funnel, and runs into the inner 
cylinder; and the proportion of this to the surface of the funnel is as 
1 to 10. By this means the actual rainfall is magnified 10 times in the 
inner cylinder, so that light rainfalls may be readily measured. 

The snowfall is often measured in the same instrument ; and in order 
to express the snowfall in inches of rain, as is usually done, the snow 
that is collected in the cylinder is melted. About one inch of rain is 
equal to 10 inches of snow; but in this there is much variation, for 
some snows are composed of very compact crystals, while others are light. 
In some cases the depth of the snow is measured and divided by 10, in 
order to be reduced to inches of rain. This is roughly correct. 

Self-registering rain gauges are made, the record of rainfall being kept 
either by means of a float that rises as the rainfall increases, or else by 
means of a pair of scales upon which the rain gauge is placed. 

Meteorological Methods and Results. — At present, nearly every civil- 
ized nation has a weather bureau from which are issued weather maps 
and predictions. In the United States the central bureau is at Wash- 
ington, and many of the states have similar bureaus. The national 
bureau issues daily maps and other publications describing or predicting 
the weather. 

The information obtained in this way is of much value. The pre- 
dictions of the Weather Bureau are very closely followed by the masters 
of sailing vessels, and much loss of life and property has been prevented 
by this means. Predictions of excessively cold weather, and of storms, 
give much information concerning the weather changes that are liable 
to occur; and by means of the warnings farmers are sometimes able to 
prepare against unusually early or late frosts. 

For the purpose of obtaining information which shall serve as a basis 
for predictions, the Weather Bureau has stations distributed over various 
parts of the country, at which observers read the records of the several 
kinds of instruments. These observations are made at regular times 
during the day, and the results are telegraphed to central stations, 
where they are all worked over and plotted upon a map. Then, with the 



436 PHYSICAL GEOGRAPHY. 

knowledge of the changes that have occurred in the preceding days, 
and knowing what changes are liable to follow, predictions of greater or 
less accuracy are made, in some cases for several days in advance. In 
many respects these predictions are of great importance ; but in addition 
to this result of the work, we are rapidly obtaining much scientific infor- 
mation concerning the air. We are also obtaining many facts relating 
to the general climatic features of the country, and of the world. But 
in these directions much less is being done than should be ; for until 
we know more about the air and its behavior, we may not expect to 
obtain more accurate predictions. 

Upon a weather map (Fig. 46) the wind direction is plotted in the form 
of a series of arrows pointing in the direction toward which the wind is 
blowing. The temperature is also placed upon them, and lines of equal 
temperature, or isotherms, are drawn across the country. The pressure 
of the air is also graphically shown on the maps by a series of lines 
which are known as isobars, or lines of equal barometic pressure, each 
tenth of an inch being represented by an isobar. The amount of rainfall 
at the different stations is printed on the maps. Thus at a glance one 
may see the weather conditions of a whole country ; and by studying a 
series of these maps made for several successive days, one is able to trace 
the variations in weather conditions for different places. 



REFERENCE BOOKS. 

Waldo. — Modern Meteorology. (Contemporary Science Series.) Scrib- 

ner, New York, 1893. 12mo. $1.25. 
Russell. — Meteorology. Macniillan & Co., New York, 1895. 8vo. $4.00. 
Abbe. — Treatise on Meteorological Apparatus and Methods, Annual 

Report U. S. Signal Service for 1887. Part II. Washington, 1888. 

There is also a description of instruments in the first part of the Annual 

Report of the Weather Bureau, 1891-1892. 

For obtaining the Dew-point and Relative Humidity, see The Tempera- 
ture op the Dew-point, etc., U. S. Signal Service, 1889. 

For all kinds of Meteorological Tables, see Guyot, Tables : Meteoro- 
logical and Physical. Fourth edition, 1884. 8vo. Smithsonian Miscel- 
laneous Collections, Washington. $3.50. 






APPENDIX II. 

TOPOGRAPHIC MAPS. 

The study of the land is greatly facilitated by the use of maps, and 
for this reason some space may be devoted to the description of the 
more common kinds of topographic maps. By far the best means of 
representing land irregularities is the model (Fig. 266), upon which ele- 
vations are shown as elevations, so that one sees the actual land forms 
in relief, although one gains an exaggerated idea of the relation between 
the vertical and the horizontal. Unfortunately, the expense of prepara- 
tion of a model is too great for its common employment. 

In some instances, elevations are shown by means of shading, this 
being known as the hachure method. By a series of lines, the actual 




Fig. 266. 
Model of Cumberland Valley, Pennsylvania. 

elevations are made to appear to rise above the rest of the country, 
while the depressions are shown in their natural relation to the high 
land. This method is used by the United States Coast Survey in 
charting the coast line of the United States (Fig. 267), and it is employed 
in some of the European countries. Its effect is very vivid ; but one 
disadvantage is, that while the differences are shown, one does not find 
information concerning the actual elevations expressed in feet. 

The contour method is extensively used, and is employed in the large 
scale map which is now being prepared of this country. While from the 
artistic standpoint it is not so effective as the hachure method, it is 

437 



438 PHYSICAL GEOGRAPHY. 

superior to this in many respects. A contour is a line of equal elevation. 
It is the line to which the sea would rise if the land were depressed to 
the depth represented by the height of the line. If we imagine our- 
selves near the seashore, the coast line is then the contour line of 0, 
and the 100-foot contour line is that to which the sea would reach if it 
were raised just 100 feet. 

The contour map (Figs. 150, 190, 228, and Plate 25) is made upon 
a horizontal scale which varies in different cases. In this country the 
usual scale is one inch to the mile : that is, every mile of country is 
allowed one inch. No allowance is made for the vertical element of 
the country. Thus if a region of considerable irregularity is being 
mapped, an inch on the sheet is made to represent one mile in a hori- 
zontal direction. As one stands upon the side of a hill, and looks across 
a valley to another hillside at the same elevation, and a mile distant, 




Fig. 267. 

the horizontal line is just one mile in length ; but if the observer should 
start to walk from the place where he stood, to the point to which he 
looked, he would need to travel considerably more than a mile. On 
ordinary maps this greater distance is not shown ; but on the contour 
maps it is brought out by means of the contour lines. The inch repre- 
sents the horizontal mile. Each descent or ascent finds a representation 
in the contour lines ; and if they are close together, one sees that the 
vertical distance to be traveled is very great. 

There is much difference in the scale of elevation represented by con- 
tour lines. On most of the maps in the eastern part of the United States, 
every 20 feet of ascent or descent is represented by a contour line, and 
we speak of this as the contour interval. Let us suppose ourselves pass- 
ing over an irregular country. Imagine that we are to travel a dis- 



APPENDIX II. 439 

tance of one mile, in the course of which we go down into one valley, up 
the hillside and down into another valley. The entire area on the map 
would be represented in the space of one inch. If the first valley had 
a depth of 200 feet, and the contour interval were 20 feet, on the map 
representing this area there would be 10 contour lines, which would 
need be very close together, because the descent of 200 feet in the small 
fraction of a mile would necessarily be rather rapid. If the hill over 
which we pass rises 40 feet above the valley bottom, we would ascend 
over a distance represented on the map by two contour lines, — a rather 
moderate ascent. If the valley on the opposite side of the hill should 
happen to be 400 or 500 feet in depth, the descent would be ex- 
tremely precipitous; and it would be necessary to represent this steep 
declivity by so many contour lines that one would merge into the other, 
and there would be a mass of crowded lines. 

From the several sections of contour maps (Figs. 150, 190, 228, and 
Plate 25, reproduced diagrammatically), one is able to understand the 
meaning of the contour lines, and to discover the irregularities which 
they represent. 1 



REFERENCES. 

Nearly every European government is publishing a topographic map, and 
among these are to be found many excellent illustrations of land forms. In 
this country, the entire area of Massachusetts, Ehode Island, New Jersey, 
and Connecticut is now mapped, and teachers can obtain these from the 
Commissioners of the Topographic Map at the state capital. In all of the 
other states there are maps of some districts ; and copies of these may be 
obtained from the U. S. Geological Survey. During the year 1895-96 the 
Survey will issue, at a small price, a few of their most instructive maps 
with descriptive text. 

The seacoast maps of the IT. S. Coast Survey are excellent and cheap. 
The same is true of the maps of the Great Lakes, the Mississippi, and the 
Missouri. A very important pamphlet (" The Use of Governmental Maps in 
Schools," Davis, King and Collie, Holt & Co., New York, 1894, $0.30) has 
been prepared for the purpose of indicating useful topographic maps. The 
methods used in making the maps of the Geological Survey are described in 
Gannett's Manual of Topographic Methods, Monograph XXII., U. S. Geo- 
logical Survey, Washington, 1893. 4to. $1.00. 

1 Specimen maps may be obtained from the U. S. Geological Survey. 



SUGGESTIONS TO TEACHERS. 

In the preparation of this book, the endeavor has been to state the 
subject in a purely descriptive manner. Nevertheless, the best way to 
learn physical geography is not to read about it, but, so far as is pos- 
sible, to work out the points for one's self. Not merely does the labo- 
ratory method teach the subject better, but it trains the mind of the 
student in a far more valuable way than is done " merely by acquiring 
information from a book. The following notes are appended merely as 
suggestions concerning the way in which simple laboratory methods may 
be introduced. There is very little necessary expense attached to the 
introduction of these methods ; but of course by the acquirement of other 
and more expeiisive materials one can improve the teaching almost with- 
out limit. 

Each teacher will need to work out the details of the problem for him- 
self; for the environment, the available materials, the time that can be 
devoted to the subject, etc., are so variable that at present it would be 
difficult to outline a course of even general value. I would urge upon 
every teacher the importance of introducing some laboratory work ; for 
it will stimulate the interest of the student, particularly if he is brought 
in contact with the real phenomena of nature. The land and the air 
are always available and full of lessons : to some, the ocean or the lake 
shore may also be within reach. I am so much interested in having 
these methods introduced that I invite teachers to correspond with me, 
if I can aid them in obtaining materials for teaching purposes. 

Chapter I. — Laboratory work in illustration of this chapter is not 
easy. Still, the best way which I know to give the student a clear idea of 
the relation of the several members of the solar system, is to have each 
student construct a rough model of it. This can readily be done by means 
of fine wire and pasteboard. By merely coiling the wire on the desk, each 
of the orbits can be made in its proper relation to the others. Then each 
planet can be made from pasteboard, the size representing a slice cut 
along the equatorial diameter. In order to have this produce the most 

440 



SUGGESTIONS TO TEACHERS, 441 

good, the scale, or relative sizes and distances, should be true to nature. 
Upon these orbits, the bodies can be made to revolve and to rotate, so 
that some idea may be obtained concerning the relative movements of 
the bodies. The relation of the moon and earth may be studied in the 
same way. 

In order to show the movements of the earth and the cause of seasons, 
an excellent method is to construct an orbit of wire and cause a sphere 
to move around it, the sphere rotating as it revolves. There are various 
ways in which this may be done; a permanent orbit may be constructed 
in the schoolroom, and a large ball, or better a globe, may be carried 
around it, each student being allowed to stand near the center, as if he 
were in the position of the sun. Each student might be allowed to con- 
struct a smaller orbit and study the earth movement himself. Celluloid 
spheres are very inexpensive, and upon them the continents may be 
roughly outlined, while an axis is passed through them to represent the 
position of the poles. An exercise or two conducted along lines some- 
thing like the above will do more to teach the students the relations of 
the bodies of the solar system than a score of lessons from the book ; 
and many students go through a course in astronomy without a proper 
conception of the solar system. The teacher will see many means of 
adding to this if more time can be spared. Thus it is possible to show 
the relations of the comets to the solar system ; the immensity of the 
distance to the stars ; the size of the sun ; aphelion and perihelion ; 
apogee and perigee, etc. 

Chapter IT. — The teacher of physics will find many opportunities for 
illustrating this chapter by laboratory methods. Thus the various effects 
of heat and light are capable of very graphic illustration. Convection 
may be illustrated by heating dust or smoke-filled air in a cylinder. 
Refraction is readily shown by the prism, and nearly all of the prin- 
ciples of light and heat may be illustrated. Compression of air can be 
very readily shown. Saturation of air may be shown by placing water 
in the bottom of a cylinder ; and then if the air temperature is lowered, 
some of the water vapor may be condensed on the sides of the vessel. 
The various ways in which humidity is increased or decreased can be 
studied in detail by each student ; and they can be given hypothetical 
cases from which to draw conclusions concerning the condition of the 
air which necessarily follows. 

The difference in the length of the summer and winter days is readily 
illustrated by the use of the globe and a candle. By placing the candle 



442 PHYSICAL GEOGRAPHY. 

in different positions, so as to throw the rays at the angles at which the 
solar rays reach the earth, and by causing the globe to revolve, this is 
easily seen by the students. 

Chapter III. — In illustration of this chapter, laboratory work maybe 
introduced by stating the latitude of a place and having the students tell 
the probable temperature conditions. Then add the altitude and have 
them state what modifying effect this would have. After this the position 
with reference to the sea may be given, and each student ought to be 
able to state the approximate conditions of temperature. They could be 
given prominent cities in the world, and have for their problem the deter- 
mination of the temperature, for which purpose it would be necessary 
for the student to first ascertain their position, altitude, etc., and this 
would also serve to teach geography. On the other hand, given a set of 
temperature peculiarities, the students can determine what parts of the 
world experience them, and why this is so. The teacher can tell the 
student of differences between places on the same latitude, or of resem- 
blances between points on different latitudes, and call for an explanation 
of these. Much similar work may be introduced if the time allows ; and 
it is safe to say that not only will the interest be aroused, but the habit 
of logical thought will be improved. 

Each student can construct a daily curve from personal observation, 
particularly if a maximum and minimum thermometer are available. 
With either fictitious or actual data, they may construct a seasonal curve. 
Placing a maximum and minimum thermometer in the ground at a 
depth of one or two feet, the difference between the range of air and 
earth temperatures is very vividly impressed upon the mind. In order 
to make this even more striking, temperature observations should be 
kept at the surface of the ground, and at an elevation of about 10 feet. 
These differences are best shown in warm weather. 

A study of the isothermal charts furnishes opportunity for observation 
and deduction, particularly if Buchan's charts (see p. 84) can be ob- 
tained. The student can construct an isothermal chart from data given 
and averaged for several places, either for the state, or the country, or 
for the locality near the school. For these and other purposes in which 
maps are needed, the set of cheap outline maps published by Heath 
& Co. of Boston, or Rand, McNally & Co. of New York, are valuable. 
Maps of all the states and territories can be obtained. The data of 
temperature, etc., for these purposes may be made arbitrarily; but 
it would be better to use the tables which can be found in the Annual 



SUGGESTIONS TO TEACHERS. 443 

Reports of the Weather Bureau. In some states, as for instance in New 
York, climatic data will be found in the State Weather Reports. These 
and the national reports may probably be obtained free of cost, provided 
a statement is furnished of the object for which they are needed. One 
report will last for many years. With these data, temperature ranges 
and other illustrations may be graphically plotted by the students. The 
amount of laboratory work possible in this and other subjects far exceeds 
the time that will be available in most schools. 

Chapter IV. — After studying the general features of the atmospheric 
circulation, the students should be able to construct a summer and winter 
wind chart for the Pacific, — of course attempting only the general feat- 
ures. Upon the charts of the Atlantic, there are many problems which 
have not been mentioned in the text ; and a thorough examination of 
the wind charts will be valuable. The Challenger charts by Buchan 
(see p. 84) contain much of value on the winds of the globe. As an 
instance of how observation and deduction may be brought into the 
study, the following might be suggested as a fair question : What condi- 
tions result in the two opposite seasons in the belt where the doldrums 
and the trade winds overlap? 

The student should note the relation between wind and barometric 
conditions. The daily weather charts 1 are valuable for this study ; 
and the student can also make his own observations with barometer, 
thermometer, wind vane, etc. A particularly valuable study can be 
made with the weather maps. i$y examining a series of such maps, one 
may observe the force and direction of the winds, and the progression 
of the conditions favoring certain winds during the successive days. 2 

When studied with reference to the conditions prevailing in its home 
region, this method becomes of much value. In this way the student 
can come into the possession of a knowledge of the causes for the winds 
that are common in his section, as well as the relation of these to the 
winds of the surrounding country. Observations on approximate wind 
force and direction can easily be made by each student ; and this will serve 
as a basis for a comparative study of the daily weather maps. Before 
the map of the day is shown them, they should be able to approximately 
foretell the probable conditions, on the basis of a series of simple observa- 

1 The teacher can probably have these sent by mail to the school. 

2 The semi-daily maps are of especial value for this purpose, and some of 
them may undoubtedly be obtained by applying to the Weather Bureau. 



444 PHYSICAL GEOGRAPHY. 

tions on the wind, temperature, and pressure. Such a study will create 
a real live interest, and make the students observers of the things of 
every-day occurrence, as well as train their minds to the habit of 
drawing logical conclusions from a series of observed facts. 

Chapter V. — The study of cyclones and anticyclones receives much 
aid from the daily weather maps. On these the student will see the form 
and size of the areas, their rate and direction of progression, the amount 
and distribution of rainfall, the direction of the winds, their spiral 
tendency, the left-hand whirling, etc. He will observe how the winds 
change from day to day, and what relation they bear to the areas of 
high and low pressure. He can predict the changes and study them 
in connection with the weather of his own immediate neighborhood. 
The storm paths and their irregularities can be studied with the aid of 
the Monthly Weather Reviews. 1 From the weather predictions, and 
the printed notes on the map, the relation between the cyclonic areas and 
thunderstorms is readily seen. The Coast Pilot 2 for the fall months, 
often contains valuable material for study in connection with West Indian 
hurricanes. 

Chapter VI. — The student can be directed in the study of the forma- 
tion and movement of clouds, and their relation to rainfall and tempera- 
ture. Reports upon these observations, from time to time, will stimulate 
them to a deeper interest in cloud formation. Attention can be directed 
to the possibility of predicting weather changes by an examination of the 
clouds. This furnishes an excellent opportunity for bringing the student 
into contact with nature. A study of the rainfall charts, 3 in connection 
with those of temperature and wind, will give opportunity for the ex- 
planation of many peculiarities of rainfall distribution. Careful obser- 
vation concerning the rainfall of the place where the student lives will 
be of value in showing the irregularities in amount, as well as in occur- 
rence. Let him compare this with that of the doldrum belt. 

A sling psychrometer (see Appendix I.) may be readily constructed from 
two thermometers, and the relative humidity of the air be determined. 
From this the student can be taught to predict the occurrence of dew 
or frost for the succeeding nights. The value of these lessons will be 
greatly increased if the students are called upon for reports. If the pre- 

1 These also may probably be obtained at Washington upon application. 

2 Distributed free by the Hydrographic Bureau of the Navy Department. 
* Those recently published by the Weather Bureau are very valuable. 



SUGGESTIONS TO TEACHERS. 445 

dictions that are made are not fulfilled, perhaps the reasons will be 
apparent; and there may have been dew at one place and frost at 
another, or dew at one home and none at another. Then the explana- 
tions for these differences can be obtained from the students. 

There are few better ways to train the habit of observation than to 
tell the students to look for certain things, giving enough directions so 
that they may be led to observe. If too little guidance is given, all but 
the brightest will be appalled by the difficulties; for one of the least 
developed parts of the student mind is generally that which directs the 
eye to look for details, and then to put these details together into a con- 
nected whole. I have often noticed how pleased secondary school stu- 
dents have been when their teacher has told them to look up something, 
and with what earnestness they have worked to have correct answers. 
They like to be made to feel that they are using their own minds; 
and it is a distinct relief from the monotony of learning what the book 
says. Since they are in constant contact with the problems of physical 
geography, each day can be made to yield opportunity for observation ; 
and nothing could be more profitable than to give the class a daily task 
in observation, devoting a part of the recitation hour to a discussion of 
the results. 

Chapter VII. — Many of the suggestions 'made for the previous chap- 
ters will apply to this ; but there are many ways in which these may be 
put together for a whole. The probable conditions of weather and climate 
in various parts of the earth may be inferred by a study of the charts of 
temperature, wind, and rain. The country near the school may furnish 
illustration of local differences in climate. 

Chapter YIII. — There is of course much opportunity for an enlarge- 
ment of this subject; but it would come better under a study of zoology 
and botany. The object sought in this chapter is to point out the rela- 
tion between climate and life, and to show also that the land itself 
presents certain obstacles to the spread of life. 

Chapters IX., X., and XI. — Unless the student dwells by the sea- 
shore, there is little of value to be obtained from an attempt at observa- 
tion study in the topics covered by these chapters. The charts may be 
studied, and reasons found for the peculiarities exhibited. If the charts 
of the Challenger Reports are available, particularly those accompany- 
ing the two final volumes of Summary, there will be found much 
opportunity for laboratory study. By a careful examination of the 
charts of the ocean bottom, much can be learned concerning the topog- 



446 PHYSICAL GEOGRAPHY. 

raphy of that large part of the earth's surface which is submerged 
beneath the ocean. 1 

One or two visits to the seashore for the purpose of studying the 
rise and fall of the tide, the action of waves, the distribution of life 
along the coast, etc., will be of great value. Even upon the shore of a 
lake some of these features may also be illustrated. The distribution of 
cold and warm water by the ocean currents, furnishes much opportunity 
for study in connection with climate. 

For tides, the "Tide Tables" (see reference at end of Chapter XL) 
give data for a very interesting study. The rise and fall of the tide is 
stated for various places ; and if the student is told to construct a dia- 
gram similar to Fig. 86, which is based upon these tables, he will learn 
much about the rise and fall of the tide. At the end of that book the 
phases of the moon and the times of perigee and apogee are stated, so 
that the reasons for many of the more important tidal variations will 
become apparent after a little study and thought. Taking the various 
stations for which the tidal predictions are tabulated, and locating them 
upon a map, one sees the geographic reasons for the difference in tidal 
height from place to place ; and given a place with a certain geographic 
location, the student can apply these principles to the approximate deter- 
mination of the tidal conditions. There is no better way to impress 
upon the student the peculiarity of tidal movement, than to have him 
laboriously construct a chart of these movements. For students of the 
interior, this is less important than for those who dwell near the sea. 

Chapter XII. — There is almost no limit to the opportunity for field 
and laboratory study upon the topics briefly outlined in this chapter, 
though it more properly falls to the province of geology. 2 Photographs 
of various phenomena, as well as lantern slides made from them, are now 
easily obtained. 3 An excellent method is to project a view upon the 
screen, and call upon students for a description of the phenomena illus- 
trated. This has the great advantage of placing an enlarged picture 
before the class, so that each student may see every feature ; and this 
does away with the necessity of many duplicate pictures with one in the 
hand of each student. Where the latter method is employed, cheap 

1 The Jones relief globe, sold by A. H. Andrews & Co., 215 Wabash 
Ave., Chicago, 111., for $100, is of great value in this connection. 

2 The study of geology could very properly be introduced here as a part of 
physical geography. 

3 The author will be glad to advise teachers who wish to obtain these. 



SUGGESTIONS TO TEACHERS. 447 

blueprints may serve admirably as substitutes. With the widespread 
introduction of electricity, it is now possible, in many schools, to make 
use of the electric lantern, which may be used in a room only partially 
darkened. Much can be done by asking the students to describe the 
features illustrated in the pictures in the book. Several phenomena are 
often illustrated in the same view. 

By far the best way to study the phenomena of the earth's surface is 
to see the actual thing; and there are usually opportunities for some 
such study near the school. In most cases the teacher can find some 
phenomena of geology, such as igneous or sedimentary rocks, fossils, 
folds, etc. The students will enjoy and profit by field excursions. 

Collections of the common minerals and rocks can be bought for a 
few dollars ; x and more will be learned by an hour's study of such a col- 
lection, than by weeks of study from the books. Some of the common 
rocks and minerals may usually be collected near the school. The 
teachers in those schools which are located within the glacial belt will 
find a storehouse of rock specimens in the clay and gravel banks. All of 
the common rocks, and many of the minerals, will often be found there. 
If the students can be sent or taken out for the purpose of making such 
collections, they will soon learn a great deal about rocks ; and this plan 
will be found admirable, even if the school has complete collections. 

Chapter XIII. — In most places the phenomena of erosion and 
weathering can be studied in the field. Rock specimens exposed at 
the surface will show the destruction in progress ; and upon exposed 
bluffs many instructive lessons may be studied. A journey in such a 
place will be found to be most profitable ; and the students will see 
important things that the majority of the world pass by without ever 
noticing. A visit to a spring may prove of value ; and if it chances 
to contain iron, or other substances, in solution, chemical action of water 
becomes something more than the mere book statement. 

In most parts of the country, wind and glacial action cannot be illus- 
trated by actual examples. Upon the lake shore, or better upon the sea- 
shore, wave action may be studied ; and in practically every part of the 
country, some form of river and rain erosion may be seen. Let the 
teacher have the students watch the rills and brooks and report upon 
the change in amount of water and sediment. This will train their 

1 Ward's Natural Science Establishment at Rochester, NY., and E. E. 
Howell, 612 17th St., N.W., Washington, D.C., have such collections. 



448 PHYSICAL GEOGRAPHY. 

powers of observation and arouse their interest; and the skillful teacher 
may make this the basis upon which to build a real understanding of 
the action of rivers. The key to success in this direction is to tell the 
student only so much as is absolutely necessary, but to make him tell the 
story, not from memory of what the book says, but upon the basis of a 
series of observations which necessarily lead to these conclusions. 

It is not necessary to find illustrations of all phenomena in the field, 
though the more, the better ; but the object is to teach the student how to 
see for himself, so that he may see other illustrations whenever he hap- 
pens to come upon them. Where it is not feasible to study the phe- 
nomena in the field, photographs or lantern slides make a fair substitute. 

Chapter XIV. — With a set of Physical Maps 1 of the continents, 
there is opportunity for study of the grander features of the land. These 
are much more naturally shown upon a relief globe. 2 The distribution 
of mountains, continents, seas, etc., are there shown very vividly. Par- 
ticularly is this the case in the second model, for here the ocean waters 
are not present to obscure the topography of the bottom of the sea. 

The larger features of the United States may be studied on the 
nine-sheet contour map published by the U. S. Geological Survey; 
and also upon the smaller shaded relief map published by the same 
bureau. 3 Better still, if the school can afford it, a model of the United 
States should be obtained. 4 With these aids, a good knowledge of the 
geography of the world can be obtained, and at the same time much 
training be gained, for the teacher will find ample opportunity to suggest 
problems for the pupil to study and answer. 

Chapter XV. — In many parts of the country, particularly within 
the glacial belt, two types of river valley may be seen within a short 
distance of the school ; and everywhere in the field it will be possible 
to see illustration of some stage of river-valley development. The teacher 
can make such an excursion, or series of excursions, the basis for an ex- 
pansion of the subject of river-valley development. Where these features 

1 The Kiepert maps are sold by most large dealers in school supplies. 
Eand, McNally & Co. of Chicago and New York advertise a similar set. 

2 Such as that sold by Eand, McNally & Co. of New York, or the Jones 
globe (see suggestions for Chapter XL). 

3 The latter accompanies the Thirteenth Annual Report of the Survey. 

4 E. E. Howell, 612 17th St., N.W., Washington, has a model of the 
country for $125, and a smaller one for $25. 



SUGGESTIONS TO TEACHERS. 449 

are not well illustrated, recourse may be had to photographs or lantern 
slides. 

A study of topographic maps will be found of great value in this 
connection, as well as in ' illustration of the features described in the 
following chapters. For suggestions concerning the special maps needed, 
and their use, see the pamphlet by Davis, King, and Collie, referred to 
at the end of Appendix II. 

Chapter XVI. — Here again there is the possibility of finding illus- 
trations in the field, and a certainty of finding them in photographs and 
slides. The U. S. Geological Survey topographic map of Niagara (free), 
and the Lake Survey map of the same (United States Engineer Office, 
34 W. Congress St., Detroit, Michigan; $0.20), are very useful. The 
latter bureau publishes a number of charts of the Great Lakes ; and on 
the Geological Survey maps, notably those of New England, many 
illustrations of glacial lakes and swamps will be found. The Mississippi 
delta is well illustrated on the U. S. Coast Survey chart 194. For flood- 
plain peculiarities, see particularly the maps published by the Mississippi 
and Missouri River Commission, whose headquarters are at St. Louis, 
Missouri. For facts concerning these maps see the pamphlet by Davis, 
King, and Collie, referred to at the end of Appendix II. 

Chapter XVII. — The effects of glaciers upon the surface of the 
land may be partly inferred from the study of a series of topographic 
maps of places within the glacial belt, and a comparison of these with 
some from outside of this belt. This may be very well supplemented 
by views from the two regions ; and then, if the school is situated 
within the glacial belt, by excursions 1 to glacial deposits. These will 
be of great value for the illustrations of many points. In all of these 
cases, the teacher should have the students observe as much as possible, 
and should avoid telling them things which they ought to be able to see 
for themselves. 

Chapter XVIII. — A teacher who has given no attention to the sub- 
ject, will be astonished to find how many lessons can be learned by an 
hour's tramp on the shore of a lake or the ocean. The beaches and cliffs 
are full of interest ; and on some ocean coasts, as well as on most lake 
shores, there will be found numerous instances of the minor coastal feat- 

1 The number of excursions suggested may seem excessive ; but it is 
assumed that no one school will be so favorably located as to make it possible 
to study all of these phenomena in the field. 

2g 



450 PHYSICAL GEOGRAPHY. 

ures, such as bars, spits, and possibly small deltas. This kind of work 
may very advantageously be supplemented, or if necessary be replaced, by 
a study of the admirable charts of the American coast, which are sold 
at a very slight cost by the U. S. Coast Survey at Washington. Some 
of these charts should be in every school where physical geography 
is taught. For those who dwell near the Great Lakes, the charts of 
the Lake Survey (sold at $0.20 a sheet) will be found very valuable aids 
to the study of shore lines. 

Chapters XIX. and XX. — To most students the subjects treated in 
these chapters are inaccessible, and they must be studied upon maps, 
models, and photographs. Unfortunately the demand for materials for 
laboratory instruction in geology and physical geography, has not yet been 
sufficient to warrant the preparation of cheap illustrative models of such 
phenomena as these. In schools where modeling is done, many valuable 
lessons could be taught by having each student illustrate these changes 
by the actual construction of models ; and a well-constructed series 
would undoubtedly find ready sale. As soon as the rational method 
of instruction is introduced, and there is a strong demand for 
new and additional material, it will undoubtedly be supplied. In the 
meantime it will be necessary for the teacher to make use of the 
only material that is at hand; namely, maps and photographs. Much 
can be learned from a carefully selected series of these ; and some of the 
schools will be situated near or among the mountains, so that, in these 
cases, excursions may be made for the purpose of studying some of the 
mountain peculiarities. In many parts of New England, in the Catskills, 
and in the entire Appalachian belt, the opportunity for this kind of 
illustration is excellent ; and if the teacher will take the trouble to look 
about him, he will find numerous interesting lessons. For instance, 
along the eastern base of the Appalachians, there exist two sets of moun- 
tain ranges, the very ancient series now reduced to low, rounded hills, 
and the younger, but still old, and relatively high Appalachians. 
Among the Cordilleras, there is an abundant opportunity for the study 
of mountains, and in many places of volcanoes also. 

Chapter XXI. — The teacher will be able to illustrate these features 
also; for by looking about him, he will find a variety of land forms, and 
among these will be found illustrations of importance. They may be 
merely plains or swamps, or they may be mountains. For the teacher 
who looks with an open eye, there is abundant chance for the discovery 
of illustrations of the relation between structure and topography. It 



SUGGESTIONS TO TEACHEBS. 451 

would not be necessary to take excursions to every place ; but an admi- 
rable method is to request the students to visit some of the places and 
report upon them. This method has been tried with good success, the 
students being sent out in squads to examine and report upon land or 
rock peculiarities, at times outside of the regular school hours. There 
are many photographs and maps which may be used in illustration of 
this chapter. 

Chapter XXII. — The teacher will find it possible to expand this 
subject in connection with the study of history and geography. Indeed, 
»hroughout physical geography there are numerous points which could 
properly be made to serve in the teaching of these subjects. Much good 
Can be done in geographic teaching by showing that, in many cases, features 
of geographic importance are not arbitrary, but have their origin in phys- 
ical causes. Most of us have learned that England is a great country, 
that it manufactures this and that, etc. ; but the fundamental reasons for 
her greatness are not ordinarily presented. We learned to bound Switzer- 
land or France, but did not learn what these boundaries meant. We 
learned the size, position, and industry of Philadelphia, but did not find 
out the reasons. If we had been told the causes, the isolated fact would 
have been more easily retained; for the average mind learns unconnected 
facts with much less ease than those which are philosophically related. 

Chapter XXIII. — This chapter is mainly intended to be one of 
information; and while abundant opportunity exists for laboratory 
work, it does not seem so essential, or so easily obtained, as in the pre- 
ceding chapters. Indeed, the teacher who follows the foregoing sug- 
gestions will probably find that the main difficulty lies in the fact that 
too much is suggested. 

Appendix I. — No part of meteorology is better capable of furnishing 
illustration by laboratory methods. The various instruments can be 
placed by the teacher, and the class be taught to make regular observations, 
just as is done at any meteorological station. These can be plotted upon 
cross-section paper, to illustrate ranges in temperature, weather changes, 
etc. By this means each of the more important instruments may be 
understood, and a knowledge obtained concerning the results of their 
use. A series of weather maps for successive days can be furnished 
each student for study, and for statement concerning the conditions and 
changes illustrated. Much interest in the subject will be aroused by 
having the weather map posted in some conspicuous place; and each 
student can be taught to see upon what basis the weather predictions 



452 PHYSICAL GEOGRAPHY. 

are made. Indeed, the students may make their own weather maps 
and weather predictions. Furnished with outline maps of the United 
States, 1 each student can plot temperature, pressure, wind direction, 
etc., for various places, from observations which the teacher furnishes 
from a map. After making one or two of these, the student will be 
in a position to thoroughly understand weather maps. Let the teacher 
take a series of weather maps for successive days, and have the class 
plot upon their maps the conditions there recorded. After two or three 
have been finished, each member of the class ought to be able to make a 
fairly close prediction of the general weather conditions of the country 
for the next day. They might even embody these predictions upon 
another map. Not only will these methods teach students how to use 
weather maps, but the mind is put to work imagining and drawing con- 
clusions from a series of facts. 

Weather maps are readily obtained free of cost from the United States 
Weather Bureau, at Washington ; and, in some states, a teacher who is 
willing to maintain voluntary observations may obtain the more common 
instruments from the State Weather Bureau. A set of the really neces- 
sary instruments is not so very expensive, and some, such as the ther- 
mometer and barometer, may also be used in the physical laboratory. 

Appendix II. — There is no better way to teach the student the mean- 
ing of the topographic map, than to have him make one of a small area. 
Moreover, it impresses the meaning of elevations in a way that no other 
in-door method can do. In the making of models and maps, there is a 
training in the appreciation of proportion, in constructive imagination, 
and in the grouping of facts, that is most valuable, and is usually not 
obtained by the student. No one should be allowed to go through the 
secondary school without having some development of the " topographic 
sense." I have known educated people who have lived in a place for 
several years without having the points of the compass in mind, who 
have had no idea of the direction to a neighboring place to which they 
have gone by train or wagon, and whose estimate of distance is simply 
ridiculous. Particularly is this true of women : for most men, by contact 
with the outer world, learn by experience what they might easily have 
been taught in school, while the majority of women get little of this 
training, even by experience. 

1 Such as those sold by Eand, McNally & Co. , of Chicago, and Heath & 
Co., of Boston, at the rate of a few dollars a thousand. 



QUESTIONS UPON THE TEXT. 

In the following questions, no attempt is made to include all that 
could possibly be asked, but rather to ask the most important, and indi- 
cate what class of questions seems best calculated to produce the most 
desirable effect, both in interesting the student, and in drawing from 
him what he knows. The questions frequently ask for a general view 
of the subject ; and it may often be necessary for the teacher to ask the 
pupil other questions which shall aid in obtaining a thorough answer. 
An excellent kind of question, is one calling for more than a mere 
answer from the text, but rather one in which the student groups 
things, partly from his own mind, and partly from the book ; such, for 
instance, as asking the application or bearing of a point treated in the 
book. The questions are arranged under sections corresponding to those 
of the book, and usually follow the order of presentation of the subject. 

CHAPTER I. 

The Earth as a Planet. Pages 3-22. 

Form of the Earth. — Of what is the earth composed ? What is its 
form? What irregularities are there on the surface? What are the 
differences between the elevation of the land and the depth of the ocean? 
What is the area of land and water ? What is the depth of the atmos- 
phere ? 

The Solar System. — What are the five classes of membei's? 

The Sun. — How does the sun differ from the other members of the 
solar system ? What does the spectroscope reveal ? What are the three 
parts of the sun? The characteristics of each? What are sun spots? 
What are the movements of the sun ? 

The Planets. — What are the important features of Mercury? Of 
Venus ? Of Mars ? Of Jupiter ? Of Saturn ? Of Uranus ? Of Neptune ? 



Asteroids. — What are these ? 



453 



454 PHYSICAL GEOGRAPHY. 

The Earth. — What reasons have we for believing that the interior is 
highly heated? What is the probable condition of the interior? What 
are the movements of the earth? What are the peculiarities of its 
revolution ? What is the cause of the seasons ? 

The Moon. — What are its movements ? What is perigee? Apogee? 
Why is one side of the moon never seen from the earth? What are the 
probable conditions on the moon ? 

Comets, Shooting Stars, and Meteors. — What are comets? How do 
they move? What is the origin of meteors? Why do they glow? 

The Stellar System. — What is the probable number and distance of 
the stars? How are they arranged? What and where are nebulae? 

Symmetry of Solar System. — What points of symmetry are noticed? 
What are the distances between the members ? Illustrate. 

The Nebular Hypothesis. — State it. 

Verification of the Nebular Hypothesis. — What points are there tend- 
ing to verify this hypothesis? What is the probability of its truth? 

CHAPTER II. 

The Atmosphere. Pages 23-42. 

General Statement. — What variation is there in the density of the 
air? What gases compose the atmosphere? What is dust in the atmos- 
phere ? Water vapor ? What is the importance of the atmosphere ? 

Light. — What is the source of our light ? Of what is white light 
composed? What is diffusion of light? Selective scattering? What 
effect upon light is produced by dust? What is the cause for the sunset 
color? What is reflection ? Give illustration. What is mirage? Loom- 
ing? Refraction? What is the cause of the rainbow? What is the 
halo? The corona? What is absorption of light? Why are bodies 
transparent, translucent, and opaque? Why are some objects colored? 

Electricity and Magnetism. — What are the indications of terrestrial 
magnetism? How is atmospheric electricity made apparent ? What is 
lightning ? Thunder ? Heat lightning ? 

Heat. — What is the source of heat ? How do different bodies behave 
toward it? What interferes with its passage through the atmos- 
phere? Why does the ocean surface remain relatively cool? What is 
latent heat ? Why does the land become warmer than the ocean ? 
How is the atmosphere warmed? What is radiation? Conduction? 



QUESTIONS UPON THE TEXT. 455 

Convection ? What is the importance of convection ? What are the 
differences in heat effect and their results ? What is the effect of rota- 
tion on the temperature of the air? Of revolution? How does this 
differ in various parts of the earth ? What are the reasons for the short, 
cold days of the temperate latitude winter ? What is the normal varia- 
tion or range in temperature during the year ? How does this differ iD 
the several zones, — tropical, temperate, and arctic ? 

Moisture. — What is evaporation? What is saturated air? In what 
places is the air naturally driest ? Why do winds favor evaporation ? 
How does temperature effect evaporation? What is absolute humidity? 
Relative humidity ? Dew-point ? What is the effect upon humidity 
caused by oceans ? By tropical heat ? By elevation ? By descent of air 
from higher altitudes? By the passage of air currents from warm to 
cold regions ? From cold to warm ? By the rising of air ? What are 
the effects of variations in humidity? 

Pressure. — In what two ways does the air pressure vary? 

Effect of Gravity. — What is its effect upon the atmosphere ? 

Effect of Rotation. — What important effect upon moving bodies of air 
and water is produced by the earth's rotation ? State the reason. 

CHAPTER III. 

Distribution of Temperature. Pages 43-67. 

General Statement. — What is the normal distribution of temperature 
from equator to pole ? What are the normal seasonal and daily ranges 
or curves ? How are they interfered with ? 

Effect of Atmospheric Movements. — In what ways do the atmospheric 
movements modify the temperature ? 

Influence of Oceans. — Why are the ocean temperatures more equable 
than those of the land? What is the effect of the oceanic circulation in 
this respect? How does the temperature change from seashore to in- 
terior ? From tropical to arctic regions ? 

Effect of Topography. — How does the temperature on the hills differ 
from that of the valleys ? How does it differ on the north and south 
sides of hills ? Why are mountain tops eolder than lowlands ? What 
does this show as to the behavior of heat ? 

Seasonal Temperature Range. — What is an isotherm ? Why are iso- 
thermal lines not parallel to the latitude ? What is the normal temper- 



456 PHYSICAL GEOGRAPHY. 

ature range? How is this shown on the isothermal charts? What do 
the curves show ? How does the range differ in various places, — ocean, 
land, and different latitudes ? Why do not the highest parts of the 
curve coincide with midsummer? The lowest with midwinter? In 
what ways is the normal curve interfered with? 

Isothermal Charts. — Why are the isotherms of the southern hemi- 
sphere more regular than those of the northern? Why is the heat 
equator north of the geographic equator? What is the effect of the 
Gulf Stream ? The Labrador current ? How does the temperature 
distribution of the west coast differ from that of the east? Why? 
Why is the heat equator so far north in July ? Why is it farther north 
in the Atlantic than in the Pacific ? Why is the deflecting influence of 
the Gulf Stream greater in January than in July ? Why do the isother- 
mal lines change in position more in the northern than in the southern 
hemisphere? Where are the coldest places on the earth ? Where is the 
cold pole? Where are the greatest seasonal ranges in the United States? 
The least? Why ? Why are deserts places of great temperature range? 
What influence of topography is shown on the chart of New Tork ? 

Daily Temperature Curve. — What is the normal daily range ? When 
do the coldest and warmest times come? Why? How does the curve 
differ in different places ? According to season ? By accidental inter- 
ruptions ? 

Temperature Ranges. — How closely do the isotherms give the real 
temperature conditions? Illustrate by San Francisco. Where are the 
lowest and highest temperatures found ? The greatest ranges ? Where 
are the greatest and least ranges in the United States? Give an example 
of rapid change. Contrast the range of Key West and Montana. Give 
an example of great daily temperature range. 

Earth Temperatures. — What is the normal change in earth tempera- 
ture ? In the tropical regions ? The temperate ? The arctic ? How 
does the temperature of the surface compare with that of the air ? 

CHAPTER IV. 

General Circulation of the Atmosphere. Pages 68-84. 

General Statement. — Illustrate mobility of the air by its action on 
deserts. Compare with the effect of a stove. How may this compari- 
son be extended to the atmospheric circulation? What are the four 



QUESTIONS UPON THE TEXT. 457 

principal parts to this circulation? In what ways are these changes 
registered by the barometer? What is a barometric gradient? 

Classification of the Winds. — Give the classification of the winds. 
What are the planetary or permanent winds ? 

Planetary or Permanent Winds : Trade Winds. — What are the trade 
winds? How and why do they move? Where are they best developed? 
Why do they produce deserts? Why do they often cause very rainy 
belts? How can the same wind produce these two opposite effects? 

Doldrum Belt. — What are the doldrums ? Their characteristics ? 

Anti-trade Winds. — In what direction do they move? How do we 
know of their existence? 

Horse Latitude Winds. — Where does the air come from? What are 
the characteristics of the belt? 

Prevailing Westerlies. — What is the circumpolar whirl? How do we 
know the permanency of these winds in the upper air? Of what value 
are they in the southern hemisphere? Why not also in the northern? 

Periodical Winds. — What are these? 

Seasonal Winds. — Where is the change of the season most noticeable? 
What effects ai'e produced in the atmospheric circulation near the 
tropics? What is the seasonal effect on the land? What is the mon- 
soon? Where are monsoons found? How is their influence noticed 
in the United States? How do the winds of Greenland show the 
influence of the season ? What is the effect of friction between wind 
and land ? 

Diurnal Winds: Sea and Land Breezes. — What is the cause of the sea 
breeze? When does it come? What are its effects? What is the land 
breeze? What do these winds resemble? What is the effect of the sea 
breeze in the trade-wind belt? What is the general effect of the day- 
time heat on the winds of the land? What are lake breezes? 

Mountain and Valley Breezes. — Describe the valley breeze as to cause 
and effect. The mountain breeze. Why are the former more violent 
than the latter? Where are these breezes noticed outside of mountains? 

Eclipse and Tidal Breezes. — What are these ? 

Irregular Winds. — How do they differ from the preceding? 

Accidental Winds. — What is the landslip or avalanche blast? What 
are the volcanic winds? The waterfall breeze? 

The Nature of Winds. — What is the real nature of the wind? 
What causes introduce a vertical movement? What are the possible uses 
of the internal work of the wind? 



458 PHYSICAL GEOGRAPHY. 

CHAPTER V. 
Storms. Pages 85-106. 

Cyclonic Storms. — What is a storm? What are some of the causes of 

storms? What are the two kinds of cyclonic storms? 

Hurricanes: Description. — Where do the hurricanes begin? The ty- 
phoons ? What changes are noticed as the storm nears and passes over a 
place ? What is the eye of a storm ? How is the air moving in the storm ? 

Effects. — What is their effect upon vessels? Upon the coast? State 
some instances. 

Path. — What is the natural path in the North Atlantic ? How do 
they sometimes diverge from this? What is their path in the Pacific? 
South of the equator? What is their size ? Where are they most violent ? 

Time of Occurrence. — When are they most common in the northern 
hemisphere? In the southern? What is the line storm? 

Cause. — What are the facts to be accounted for? Why may we 
expect that the heat of the tropics is the cause for their beginning? 
What would account for the whirling? What reason is there for the 
greater influence of right-hand deflection in certain seasons? Why 
should they be confined to the ocean? What is the effect of condensation 
of water vapor? Why do the storms lose energy when they have passed 
beyond the tropics? What is the explanation of the path? Describe the 
hurricane. State its cause briefly and clearly. 

Temperate Latitude Cyclones : Resemblance to Hurricanes. — How do 
they resemble hurricanes? 

Differences from Hurricanes. — How do they differ in general behavior? 
In time and place of development ? In path ? What is the usual path ? 

Effects. — Where do they occur? What are their effects in the United 
States? 

Winds. — How do these vary? What changes occur as the storm 
passes? What is the sirocco? The foehn? The chinook? The bliz- 
zard? The norther? 

Anticyclones. — What is their cause? What are cold waves? What 
are the accompanying conditions of winds? 

Cause. — What was the former theory? What objections can be urged 
to it? State a possible explanation. What is the reason for their paths? 

Secondary Storms: Thunderstorms. — Where do they occur? Under 
what conditions? What is the cause for the thunder cloud? Its form 



QUESTIONS UPON THE TEXT. 459 

and features? What is their relation to cyclonic storms? Their path? 
What is a cloud burst? Describe and discuss the thunderstorm. 

Tornadoes and Waterspouts. — What are the form and characteristic 
features of the tornado? Their effects? The area covered and time 
occupied? In what respect do they resemble thunderstorms? What is 
the cause? What is a waterspout? 

CHAPTER VI. 

The Moisture of the Atmosphere. Pages 107-123. 

Dew. — What is the cause of "sweat" on a pitcher of ice water? How 
does this resemble dew formation? At what temperature and time will 
this occur? What conditions especially favor the formation of dew? 
Why does dew occur more readily in valleys than on hilltops ? What is 
the main cause for dew? What other causes also aid? 

Frost. — What is frost ? What prevents it ? 

Fog. — What is fog ? What is the cause for ocean fog ? What is 
valley fog? In what other ways may fog be caused? What is the 
relation of dust to fog? 

Haze. — What is haze? Its cause? 

Mist. — What is mist? 

Clouds. — Of what are clouds composed? Under what condition are 
they formed? Give the classification of clouds. Describe the cirrus ; the 
cirro-stratus ; cirro-cumulus ; cumulus ; cumulo-stratus ; stratus ; nimbus. 

Rain. — What is the cause of the drop ? Under what conditions is 
rain caused? What relation does it bear to clouds? 

Snow. — What is snow ? The difference between snow and rain? 

Hail. — What is hail? 

Distribution of Rainfall in the World. — What do we mean by rain- 
fall? Why are there differences according to altitude and latitude? 
What is the cause for variation in tropical regions ? What is the effect 
of steeply rising mountains? What are the two main causes for deserts ? 
What are the rainfall peculiarities within the belt of calms ? How does 
the rainfall vary from coast to interior ? 

Distribution of Rainfall in the United States. — What are the causes 
for the heavy rains of the Texas and Florida coasts? For the differences 
between the east and west coasts? What is the effect of the high west- 
ern mountains upon the rainfall of the western half of the country ? 



460 PHYSICAL GEOGBAPHY. 

Distribution of Snowfall. — Where does snow fall ? Where are glaciers 
produced ? 

Seasonal Distribution of Rainfall. — What is the effect of the migra- 
tion of the belt of calms? How do the monsoons affect the seasonal 
rainfall? What is the reason for the winter rains of Washington and 
Oregon ? For the irregularities of rainfall in the east ? 

Irregularities of Rainfall. — What is the normal rainfall ? How does it 
sometimes vary from this ? What are the effects of heavy downpours ? 

CHAPTER VII. 
Weather and Climate. Pages 124-134. 

Weather. — What is weather ? Climate ? 

Tropical and Arctic. — What are the weather conditions of the belt of 
calms ? Of the trade-wind belts ? Of the polar regions ? 

Temperate Latitude Weather. — What are the weather conditions on 
the northern Pacific coast? In the mountains east of this? In the 
deserts between the mountains? On the plains of Dakota, etc.? On 
the more southern plains? In the southern coastal states? In the 
northern central states ? What is the cause for the droughts ? What 
are the weather conditions of the northeastern states? What are the 
winter conditions in this belt? The summer climate? What are the 
typical weather conditions in temperate latitudes? How do those de- 
scribed differ in Europe ? In the southern hemisphere ? 

Climate. — What are the climatic belts ? Their subdivisions ? 

Tropical Climate. — What is the general climatic condition ? The 
difference between the ocean and the land? The doldrum and trade- 
wind belts? What are the differences in rainfall? What climatic 
peculiarities are caused by the monsoon condition of India? 

Temperate Climate. — What are the characteristics of the climate of 
this belt? What are its subdivisions? What is the climate of the 
western coasts? Of the eastern coasts? The interior climate? Of 
mountains? Of the inter-montane district. State the climatic differ- 
ences noticed on the parallel of 50° N. 

Arctic Climate. — What are its characteristics? 

Minor Variations. — What are some of these? 

Changes in Climate. — What two classes of evidence point to climatic 
change? What is the supposed thirty-six-year cycle? What is the 



QUESTIONS UPON THE TEXT. 461 

geological evidence of former differences in climate? What recent 

geological changes are recorded in the United States? What are the 
possible explanations of these changes ? 



CHAPTER VIII. 
Geographic Distribution of Animals and Plants. Pages 135-148. 

General Statement. — What are the life zones? What kinds of life 
occur in the several zones ? What are the differences between the life 
in fresh and salt water? 

The Ocean. — ■ What causes the wide distribution of ocean life ? What 
is the effect of temperature on distribution? Where in the ocean are 
plants unable to live ? Under what conditions do they especially thrive ? 
What is the difference between the tropical and northern animals? 

Fresh Water. — What are land-locked animals? What forms of life 
are found in fresh water ? What is the effect of change to salt lake ? 

The Land : — Effect of Temperature and Moisture. — What is the effect 
of temperature? What is the effect of arctic cold on the animals? 
On the plants ? Of the cold of high temperate latitudes ? What is 
the influence of altitude? What changes in vegetation are noticed in 
ascending high mountains ? How may this vary on the opposite sides 
of a mountain ? What are the effects of aridity ? Of great moisture ? 

Plant and A nimal Habits. — How do the seeds effect the distribution of 
plants ? What animal habits influence distribution ? 

Life Zones. — What are the great life zones and their subdivisions ? 
How do the continental zones resemble one another? How do they 
differ ? What do these differences and resemblances show ? How is this 
illustrated by oceanic islands ? In the Bermudas ? In New Zealand ? 
The East Indies ? Australia ? 

The Spread of Life. — What is the main reason for the distribution of 
land animals? What is the effect of the winds and storms? What 
animal groups are distributed by this means? What is the effect of 
ocean currents ? What animals are thus liable to be carried ? Why are 
large animals so rare on oceanic islands ? What was the effect of the 
change of climate causing the glacial period ? 

Barriers to the Spread of Life. — What is the great barrier? What 
does Australia teach us in this respect ? What other barriers are there ? 

Effect of Man. — What is the effect ? Is there any limit to it ? 



462 PHYSICAL GEOGRAPHY. 



CHAPTER IX. 

Form and General Characteristics of the Ocean. 
Pages 151-173. 

Distribution of Land and Water. — What are the main features of 
distribution of land and water ? 

Composition of Ocean Water. — What are the principal ingredients of 
salt water ? How much variation is there in salt impurities ? What are 
the reasons for this ? 

Color and Phosphorescence. — What is the natural color of the ocean ? 
Why ? Are there other colors ? What is phosphorescence ? 

Exploration of the Ocean Bottom. — What reasons led to the belief that 
animals could not live here? How can the animals exist under the 
great pressure ? What has led to the study of the deep sea ? 

Methods Used in Deep-sea Explorations : Sounding. — What are the 
objects sought ? What is a fathom ? Describe the sounding machine. 
What other facts are learned during the sounding ? 

Dredging. — Describe the deep-sea trawl. How correct a knowledge 
may we expect to obtain by dredging ? 

Topography of the Ocean Bottom : General. — What is the fundamental 
difference between the land and ocean bottom topography ? Why are 
there greater occasional elevations in the ocean ? Why greater general 
levelness ? What are the general features of the ocean bottom ? State 
some of the excessive differences in elevation in the ocean. 

The Atlantic Ocean. — What is the continental shelf? The continental 
slope? The oceanic plateau? The mid-Atlantic ridge? What are the 
features east of this ? What features are shown in a cross-section of the 
Atlantic ? 

Other Oceans. — How do the features of the Pacific correspond with 
those of the Atlantic ? What is the deepest known point in the Pacific ? 
In the Atlantic ? Compare ocean depths with land elevations. 

Topography near the Coast. — Compare this with the ocean depths. 

Temperature of the Ocean Bottom. — What are the temperature features 
of the ocean bottom near the land? How does this change with increas- 
ing depth ? What is the general temperature condition of the waters of 
the ocean bottom? How does this vary in such places as the Medi- 
terranean? The Gulf of Mexico? What is the explanation? 

Light on the Ocean Bottom. — What is the probable source of this ? 



QUESTIONS UPON THE TEXT. 463 

Materials Composing the Ocean Floor : Mechanical Sediments. — What 
are the two sources of ocean deposits ? 

Globigerina Ooze. — What is this? Where does it occur? How is it 
accumulated ? What rock resembles it ? 

Red Clay. — What is this ? Where does it occur ? What materials 
compose it ? How large an area does it cover ? 

Life in the Ocean : Pelagic or Surface Faunas. — What ocean conditions 
especially favor abundant life ? Why is the temperature uniform ? What 
conditions favor the widespread distribution of the surface animals? 
Under what conditions do they live? Do animals live in the waters be- 
tween the ocean surface and bottom ? 

Littoral or Shore Faunas. — How do the conditions in this zone resem- 
ble those of the land ? What is the effect of temperature here ? Illus- 
trate. How does the food supply influence the development of these 
animals? Illustrate by coral growth. What are the habits among shore- 
line animals ? How do these vary ? 

Faunas of the Ocean Bottom. — How do the deep-sea animals show the 
effect of pressure when brought to the surface ? What forms live on the 
ocean bottom? What is the main cause for limiting their spread? 
Under what conditions do they exist? How does the low temperature 
tend to diminish the abundance of animals? What is their food sup- 
ply ? How does this also limit their abundance ? How do they obtain 
their oxygen ? What do they prove with reference to oceanic circulation ? 
How does the oxygen supply tend to limit the abundance of life ? 

CHAPTER X. 
Ocean Waves and Currents. Pages 174-191. 

Wind Waves. — What is their cause? Their form? How do they 
move ? What change is caused at the shore ? How far do they extend ? 
When are they formed? How do they act on the shore? What are 
their effects ? Their every-day action ? How may their effects be seen ? 

Earthquake Waves. — What are these ? How do they behave ? What 
are their important effects ? How far may they travel ? 

Storm Waves. — What causes tend to produce these ? Their effect ? 

Ocean Surface Temperatures. — What is the natural change from place 
to place? How may this be made to vary? What influence is noticed 
near the coast? What are the conditions in mid-ocean? Why is the 



464 PHYSICAL GEOGRAPHY. 

warm surface water so shallow ? Why are the surface temperatures so 
constant ? 

Ocean Currents : Planetary Circulation. — What resemblance is there 
between ocean and air circulation ? What reasons are there for believing 
in a planetary, oceanic circulation ? 

The System of Ocean Currents. — What is the circulation in equatorial 
regions? What is the North Atlantic drift? What becomes of the 
water entering the Caribbean ? What is the origin of the Gulf Stream ? 
Its course ? What is the Labrador current ? Briefly describe the general 
circulation of the North Atlantic. What are the conditions in the South 
Atlantic ? What is the circulation of the North Pacific ? What is the 
Kuro Siwo ? What is the circulation of the South Atlantic ? What are 
the main features of the oceanic circulation ? 

Cause of Ocean Currents. — What reasons are there for doubting the 
temperature theory ? What is the apparent explanation ? What facts 
support this? What influence has the temperature difference? What 
causes determine the course of currents ? What would be the circula- 
tion if there were no land ? 

The Gulf Stream. — What is the reason for its warmth? Its velocity? 
How does it vary in velocity? 

The Labrador Current. — What is its course? 

Effects of Ocean Currents. — What is the most important effect? 
What would result if there were no circulation? What indication is 
there of an important influence upon temperature? How much heat is 
carried? What is the influence upon rainfall? Upon sailing vessels? 
In producing fogs? Upon animal life in the ocean ? 

CHAPTER XI. 

Tides. Pages 192-203. 

Nature of the Tidal Wave. — What is the nature of the wave? 

Cause of Tides. — What is the origin of the wave? Why is the in- 
fluence of the moon greater than that of the sun ? 

Effect of the Land. — What is the natural course of the wave? What 
is the cause for its peculiar movement in the Atlantic? What is the 
change introduced in bays ? What are the peculiarities near the British 
Isles ? In the approaches to New York ? How does the height vary ? 
How may it be lessened? How may it be increased ? What is the effect 



QUESTIONS UPON THE TEXT. 465 

of the difference in the height of the tide in connected bays? What are 
tidal races? Illustrate. What is the tidal bore? 

Other Causes for Variation in Tidal Height. — What is the effect of the 
wind? Of air pressure? What are seiches? How does the relative 
position of sun and moon influence tidal height? What are spring 
tides? Neap tides? What is the influence of perigee and apogee? 
What other astronomic causes for variation are there? 

Effects of Tides. — What is their influence upon navigation? In 
changing the coast? What is their effect in estuaries? How are the 
tides utilized? 



CHAPTER XH. 
The Crust of the Earth. Pages 205-223. 

Interior Conditions. — What reasons are there for believing that the 
interior of the earth is highly heated? What was the former belief? 
The present hypothesis? What is the apparent effect of loss of heat? 

Movements of the Crust. — What classes of proofs are there showing 
the crust to be in movement ? State some of the historic proof. The 
geologic evidence. Is this a movement of the water or the land ? 

Disturbance of the Rocks. — What is the position of the rocks of the 
crust? By what means are they changed from the horizontal? What 
is a monocline? Anticline? Syncline? What are the characteristics 
of the folds in mountains? What is dip? Strike? A fault? A fault- 
plane? How does the movement take place? 

Volcanic Action. — What is a volcano? A lava flow? Volcanic ash? 
Pumice? How do volcanoes vary in their ejections? How large an area 
is covered? What are dykes? Bosses? 

Rocks of the Earth's Crust. — What are the three groups of rocks? 
What is their origin? 

Igneous Rocks. — What are minerals? What rocks are crystalline? 
How do these rocks vary chemically? What minerals occur in them? 
Why are some igneous rocks coarse grained, while others are fine. 

Metamorphic Rocks. — How do they resemble the igneous? What 
are their characteristics? Their origin? What are the common rocks 
of this group? 

Sedimentary Rocks. — What are the three subdivisions ? Which is 

2h 



466 PHYSICAL GEOGRAPHY. 

most important? How are the mechanical sediments derived? How 
are they accumulated ? What are the kinds ? How do they differ ? 

Deposition of Sedimentary Rocks. — In what position are they depos- 
ited in the ocean? What is the origin of stratification? What are the 
characteristic deposits in the sea? What are the characteristic sedi- 
mentary rocks on the land? What does this prove? How thick are the 
sediments. What does this prove? What is an unconformity? 

Consolidation of Sedimentary Rocks. — How are rocks cemented? Illus- 
trate. What are the common rock cements? 

Geological Chronology. — What is the condition of the rock record? 
What are fossils? How has a record of early life been obtained? What 
does this show? Can the age be told by fossils? What is the difference 
between age and stage? What do the names of the geological periods 
really indicate? What does the name Carboniferous mean? Learn the 
table of geological ages. The groups of animals that lived then. 

Age of the Earth. — What do the estimates show? What does geology 
show as to the age of the earth? Illustrate by Niagara and the Colorado. 
By volcanoes. By the thickness of sedimentary rocks. What are the 
two fundamental conceptions in geology? 

CHAPTER XIII. 
Denudation of the Land. Pages 224-248. 

Underground Water. — How does water find its way into the rocks? 
How does it move through them ? What is the evidence of its existence? 
How is it able to dissolve? What evidence of this is there? Why 
should some of the dissolved mineral substances be deposited? What 
effects are produced by the deposits of this in the earth? What effect 
is produced by underground water in changing minerals? 

The Formation of Caverns. — What is their origin? What are stalac- 
tites? Stalagmites? What is the origin of the natural bridge? 

Springs and Artesian Wells. — In what ways are springs produced? 
What are the conditions favoring the accumulation of artesian water? 
What rock is particularly favorable? What must be the position of the 
rocks? Why does the water rise to the surface? Why does it not rise 
above the permeable layer? What is the use of this water? 

Durability of Rocks. — How do rocks vary as regards durability? What 
is the influence of texture? What is meant by a hard rock? 



QUESTIONS UPON THE TEXT. 467 

Weathering. — What agents are engaged? What are the chemical 
changes? How do these affect the rocks? In what rocks are they most 
liable to act? What sedimentary rocks does this decay form? What 
is the most important mechanical agent? What conditions favor the 
action of this ? Where is it checked ? How do plants aid in weather- 
ing? Animals? How widespread is the action of weathering? Where 
is its action rapid? Where slow? What are the results? With what 
is weathering in combat? Which has excelled? What would have 
been the result had there been no re-elevations ? If there had been no 
other agent of destruction? What agents have aided the effectiveness 
of weathering ? What is residual soil ? Where is it important ? 

Agents of Erosion. — What are the most important of these? 

Wind Erosion. — Where is this important? What is its effect on the 
seashore? What are sand dunes? Why is wind erosion important in 
arid regions? What is its effect ? 

Rain Erosion. — When does this action commence? What is its effect? 
Where is it least important ? What is the origin of gravel slopes ? What 
is the importance of gravity? 

Percolating Water. — How does this act? How does it act mechani- 
cally? How are avalanches or landslides produced? 

River Erosion. — What tasks are rivers engaged in? What materials 
are furnished to them? How do these materials vary in amount and 
kind? In what way does the river erode? Why are most arid land 
rivers V-shaped? Why are newly begun valleys V-shaped? What 
causes them to broaden? By what means is the rate of erosion caused 
to vary? How do rivers vary? What is their most important office? 

Ocean Erosion. — How do waves act ? How are materials removed ? 
How does this affect the coast line. 

Glacial Erosion. — How does ice erosion differ from that of water? 

Denudation. — What is denudation ? Whence come the forces ? How 
do the agents interact? What has been the importance of their action? 

CHAPTER XIV. 

Topographic Features of the Earth's Surface. Pages 249-261. 

Continents and Ocean Basins. — What are the greater irregularities of 
the earth? What is the arrangement of land and water? What is the 
relative size of the continent and ocean areas? What are the more 
important features of the ocean bottom ? What is the elevation of the 



468 PHYSICAL GEOGRAPHY. 

land compared with the ocean depth? What are the most characteristic 
features of continents? Are the continent forms permanent? What 
changes are in progress ? Where is the real continent border ? 

Physical Geography of the United States. — What are the five geo- 
graphic provinces? 

Atlantic Coast Area. — What is the extent of the coast plains? What 
are the characteristics? What are the characteristics of the plain on 
the landward side of this? Of what value are these areas? 

The Eastern Mountains. — What are their features ? What are the 
two parts? The extent of the older mountains? Their features'. 
What is the relative age of the Appalachian and the more eastern moun- 
tains? What are their features? Why are they less high than the 
Andes and Rockies? What are their most important mineral products? 

The Canadian Highlands. — Where do these extend into this country? 

The Central Plains. — What are the main features of these ? Their 
extent ? How are they interrupted in places ? For what are they valu- 
able ? Why are they not forested ? 

The Cordilleran Area. — What are its main features? What are the 
features on the eastern base? In the Rocky Mountains? West of 
these ? In the Sierras ? At the western base of these ? On the Pacific 
coast ? Why are these mountains so high ? What are the indications 
of intense denudation? What is the condition of volcanic activity in 
this region? Elsewhere on the continent? What is the importance of 
this area in mineral production? 

The Drainage of the Country. — (See map.) Into what oceans does the 
water drain? What part drains to the Arctic? Through what river? 
What to the Pacific ? Through what large rivers ? What two important 
rivers enter the Gulf ? What is the condition of the Appalachian drain- 
age? What are the features of the St. Lawrence drainage? 

The Shore Line. — What is the general form of continents ? What are 
the main features of the Atlantic coast line ? Of the Pacific ? 

CHAPTER XV. 

River Valleys. Pages 262-284. 

General Description. — What is a river? What are the general char- 
acteristics of river valleys ? What is a river system ? A divide ? How 
do rivers differ ? What was the former belief concerning river valleys ? 
What do we now know to be their origin ? 



QUESTIONS UPON THE TEXT. 469 

Development of River Valleys. — What actions combine to produce the 
valley ? What is base level ? When in river development does erosion 
exceed weathering ? When does this cease ? What would be the ulti- 
mate result ? What is the valley-form in youth ? In maturity ? Where 
is the development earliest and most rapid ? How may the valley-form 
vary in different parts of the course? What is the influence of rock 
structure? Of sediment load? Of arid conditions? What would the 
canon valley show as to age ? What evidence is there that weathering is 
in progress? What other features of youth are there? How do the 
number of tributaries show age ? What is the condition of the divide ? 
What happens when vertical erosion ceases ? What is the condition of 
the river in this stage of maturity? What stage have most valleys 
reached? What characteristic features have led. to the division of the 
river course into three parts ? Why cannot this be considered universal ? 
How may the rate of development vary ? What would be the difference 
between a valley on a plain and on a plateau ? How may the climate 
influence this? Why do gorges remain so long in mountains? What 
would be the effect of a mountain lake ? What is the origin of the broad 
valley in high mountains ? 

Adjustment of Streams. — What is a consequent stream course? How 
may this change as the river develops? What is mature adjustment? 

The River Divide. — Are these permanent ? How may they change ? 
What is the law of monoclinal shifting ? How may divides be suddenly 
changed ? 

Accidents to Streams. — What would be the condition if no accident 
interfered with river development? In what different ways do these 
accidents affect stream valleys ? What are composite streams ? 

Land Movements. — What are the three kinds ? What would be the 
effect of a general uplift? Along the seashore? Is this rejuvenation 
common ? What would be the effect of depression ? How is this illus- 
trated on the eastern coast? How will folding influence the streams? 
What are antecedent rivers ? How may the river course be changed 
by mountain growth? What features are introduced? 

Climatic Accidents. — What are the effects of a change to a condition 
of dryness? What is an arroya? What are withered or shrunken 
streams ? What are the first effects of glaciation ? How are the lakes 
formed along the margin ? Give instances. How may stream courses 
be changed ? What are the results ? What effects are produced by vol- 
canic action ? By avalanches ? Why is the old-age stage not reached ? 



470 PHYSICAL GEOGRAPHY. 

CHAPTER XVI. 
Deltas, Floodplains, Waterfalls, and Lakes. Pages 285-305. 

Deltas. — Where are delta deposits made ? What is the alluvial 
fan? What conditions favor delta formation in the ocean? Why are 
lakes favorable places for these ? How does the river flow over the 
delta? What are distributaries? How does the delta grow ? 

Floodplains. — Where are these found? What causes floodplains 
among mountains? What is the most common cause for floodplains? 
How may they merge into deltas? What effect would be produced by 
tilting the land? From changes of climate? What are the character- 
istics of floodplains? What is the course of the stream? What are 
oxbow cut-offs? How are the floodplains raised? How does the flood- 
plain material move down stream? What is the effect of the floodplain 
upon tributaries ? 

Waterfalls. — What is their origin ? What cause has produced most 
of these? What was the origin of Niagara? Its history? The falls of 
St. Anthony ? What other causes produce falls ? What is the fall line ? 
Its importance? How may waterfalls be naturally developed? What 
is the most common position of the rocks in which these are developed? 
What is the origin of such rapids as those of the Colorado? 

Lakes. — How do they differ? What relation do they bear to rivers? 
How may they be produced ? What is the most common cause ? What 
other accidents produce lakes? What are original lakes? How may 
lakes be naturally developed? How permanent are lakes? How are 
they destroyed ? Which of the processes is the more important ? Why ? 
Under what conditions may cutting at the outlet become of importance? 
Illustrate one of these by Niagara. What is the effect of evaporation ? 
What have been the changes in the Great Basin ? 

Swamps. — What relation do these bear to lakes? How' does the 
change take place? In what other ways may swamps originate? 

CHAPTER XVII. 
Glaciers. Pages 306-327. 

Cause of Glaciers. — What is a glacier? How does it form? What 
determines the terminus? Where are conditions found which favor 
their formation? What are the kinds of glaciers ? 

Alpine or Valley Glaciers. — Where are these found? What is the 
snow field? How does the glacier receive its supply? How does it 



QUESTIONS UPON THE TEXT. 471 

move ? What are crevasses ? What is an ice fall ? What are the causes 
of irregularities on the surface ? How is the glacier supplied with rock 
material? What is the lateral moraine? The medial moraine? The 
ground moraine ? The terminal moraine ? What is the origin of the ice 
cave ? What are the characteristic features of the valley glacier ? What 
are the characteristics of the glacier at the foot of Mt. St. Elias. 

Continental Glaciers. — Where are these now found? How extensive 
are they ? How thick are these ice sheets ? What are the features of 
the Greenland glacier ? What are nunataks ? 

Icebergs. — What is floe ice ? How are icebergs formed ? How far do 
they journey ? How much is below water ? How high are some bergs ? 

Glacial Period : Area covered by Ice. — What recent changes of climate 
have taken place ? What was the effect ? How extensive was the glacia- 
tion? What were the conditions in northeastern America? In Europe? 
Were these two areas connected? What were the conditions in Asia? 
In western America ? What do we know about the cause for this change 
in climate? How long ago did the ice sheet disappear? 

Terminal Moraine. — How did the glacier resemble the Greenland ice 
sheet? What was accumulated at its margin? Where is the terminal 
moraine? What are its features? 

Formation of Soil. — What is till or boulder clay ? What are its 
characteristics ? What are the signs of a scouring action ? How deep 
is the soil? What other kinds of soil were left? 

Formation of Lakes. — How were temporary lakes formed? AVhat 
effect was produced in the Red River valley ? What was the size and 
extent of this lake? What is the proof of this? How were lakes formed 
by the deposit of glacial drift ? How were rock basins formed ? What 
large lakes were produced by the action of the glacier? 

Formation of Waterfalls. — How were the stream courses interfered 
with ? Why are the new valleys gorges ? Why were waterfalls caused ? 
What was the general effect of the ice upon the topography ? 

CHAPTER XVIII. 

The Coast Line. Pages 328-349. 

General Statement. — What changes are taking place? What agents 
are at work ? How do lake and sea shores resemble one another ? 
Effect of Elevation. — What are the effects of this ? 
Effect of Depression. — What are the effects of this? What would 



472 PHYSICAL GEOGRAPHY. 

result from the depression of the land bringing sea level to the plaoe 
occupied by the student? What is shown on the coast of Maine? 
Where else is this also shown? What are the two general types of 
coast? Why? Give illustrations. 

Effect of Sediment. — What becomes of most of the sediment? When 
the sediment supply is too great, what becomes of it ? Why are sand bars 
produced in the sea? 

Effect of Waves and Currents. — What are these doing on exposed 
coasts ? Give some illustration from the English coast. From the 
American. What are bars? Spits? Hooks? How does the effect vary 
with the hardness of the rock? What is the tendency of the wave 
work? How are lagoons formed by beach barriers? What is the 
natural form of the beach ? 

Effect of Plants. — What is the effect of seaweeds ? Of the mangrove? 
Of the marsh grasses ? 

Effect of Animals. — Under what conditions may corals live? Why 
are they absent from some tropical coasts ? What do they build? What 
are barrier reefs? Keys? Atolls? Why are these above sea level? 
What is the Darwin theory for atolls. 

Changes in Coast Form. — What are some of the causes for change ? 
What are some of the recent changes on the eastern coast ? 

Islands. — How do these vary ? What are the classes ? What are 
the classes of oceanic islands? Where are these represented on the 
coast? What are the causes for most of the islands? What becomes of 
islands if left to the waves? Illustrate. 

Promontories. — What is the difference between capes and promonto- 
ries ? What are the causes for some of the larger promontories ? What 
is the origin of the Nova Scotia peninsula ? Florida? Sandy Hook? 

Lake Shores. — What are the features of these? How are capes and 
islands formed in them? What is the origin of the Thousand Islands? 
What part of the seashore do most lake shores resemble ? 

Fossil Shore Lines. — How are these formed ? What are their features ? 
How durable are they ? Give some instances of these. 

CHAPTER XIX. 

Plateaus and Mountains. Pages 350-369. 

Plateaus. — What is a plateau? How does it differ from a plain? 
With what are they associated? Where are they found? What large 



QUESTIONS UPON THE TEXT. 473 

plateaus are covered by lava? What is the climate of the plateaus of 
the west ? How do the western plains differ from the prairies ? What is 
the condition of the river valleys? Why? What is the characteristic 
topography of the high plateaus? What is a mesa? A butte? 

Mountains : Characteristics of Mountains. — What is a mountain ? 
What is the origin of the features? What is a mountain system? A 
Cordillera? A range? A ridge? How do they resemble one another? 
What is a peak? What is the origin of the peak? Of what are they 
made? How do they differ from the ridge? What other kinds of peaks 
are there? What are hills of circumdenudation ? What are interior 
basins ? Where are they found ? What is their comparative importance 
in different continents ? What is the origin of the longitudinal valleys ? 
What are parks? What is the origin of mountain gorges? What are 
passes? What is the characteristic topography in mountains? What 
are the reasons for this ? What are the features of the flora ? Why are 
mountain peaks rugged ? Upon what does the form of the peak, ridge, 
etc., depend? When are mountains most rugged? 

The Origin of Mountains. — State the contraction theory. What com- 
parison may be made concerning the wrinkling of the crust? What 
is the value of this theory? What is the history of mountain folds? 
How do mountains grow? What happens as they grow? What would 
be the result if denudation had been absent ? 

Sculpturing of Mountains. — What determines the result of this? 

The Drainage of Mountains. — What determines the drainage? What 
are the characteristics of the mountain drainage? What are longitudi- 
nal streams ? Transverse valleys ? What may be said about the origin 
of antecedent valleys ? What is the origin of mountain lakes? Their 
characteristics ? 

Destruction of Mountains. — What are the features of young moun- 
tains? Why? What happens as the age increases? What is the stage 
reached by the Appalachians? By the eastern highlands? What 
changes occur in the position of the hard and soft layers? What are 
synclinal mountains ? 

CHAPTER XX. 

Volcanoes, Earthquakes, and Geysers. Pages 370-389. 

Volcanoes : Distribution. — Where do they occur with reference to the 
sea? To mountains? Where found in North America? What about 



474 PHYSICAL GEOGRAPHY. 

their former abundance? Have they occurred in all parts of the 
world? 

Materials Erupted. — What substances are erupted ? What is the 
cause of pumice ? What are the effects of the steam ? What is a mud 
flow? How does the lava flow move? What is the extent of the lava? 
How does this differ from ash? What was the effect of Krakatoa? 

Eruptions of Volcanoes. — How do these vary as to violence ? Contrast 
the eruption of Krakatoa with those of the Lipari Islands. What is the 
case in Vesuvius ? In the Hawaiian Islands ? What kinds of volcanoes 
are the most violent ? What are the three groups ? 

Form of Cone. — How does a volcano grow? What tends to destroy 
the cone? Where are they steepest? What is their angle of slope? 
How do lava and ash cones differ ? 

Effects of Volcanic Eruptions. — What are the more important effects ? 

Extinct Volcanoes. — What happens after volcanoes become extinct ? 
What are volcanic necks ? What are dykes ? What are buttes ? Mesas ? 

Cause of Volcanoes. — What is the immediate cause ? What is the 
origin of the heat? What is the association with mountains? Why? 

Earthquakes. — Where do these occur ? What is the nature of the shock? 
What is the focus ? The epicentrum ? How does the shock travel out 
from the center ? What are the effects ? What may cause earthquakes ? 

Geysers and Hot Springs. — What is the origin of hot springs? With 
what are they commonly associated? What is the association with ore 
deposits ? What is the relation between geysers and hot springs ? Where 
are geysers found? What are their characteristics? 

CHAPTER XXI. 

The Topography of the Land. Pages 390-406. 

General Statement. — How are land forms derived ? What are the 
forces? What would be the result if denudation had been absent? 
What are the opposing forces succeeding in accomplishing ? What feat- 
ures and forces determine the complexity of the land form ? 

Constructive Land Form : By Internal Forces. — How are these compli- 
cated? What are the larger constructive forms? What is the origin of 
the coast plains ? Volcanic cones? 

By Agents of Denudation. — What constructive forms are produced by 
gravity? By wind? In lakes? By rivers? By glaciers? In the ocean? 
How are these forms modified ? 



QUESTIONS UPON THE TEXT. 475 

By A nimal and Plant Life. — State some of these. 

Effect of Rock Structure upon Topography. — How may rock character- 
istics influence the action of denudation? What are the features in high 
mountains ? In arid climates ? What influence does the stage of devel- 
opment have upon topographic form? What is the effect of uniformity 
of texture ? Of variation ? What is the effect of position ? When the 
rocks are horizontal? What are terraces of differential degradation? 
What forms result when the rocks dip gently ? What are the features 
found in traveling over such a region? What results on the seacoast? 
What happens in mountains with steeply inclined strata? When rocks 
are harder than others, what happens ? What results when submergence 
occurs ? What is the interaction of the various forces ? 



CHAPTER XXH. 

Man and Nature. Pages 407-419. 

General Statement. — How does man's present condition differ from 
that of the past? How may the subject be divided? 

Modifying Influence of Man. — State some of the ways in which he 
modifies nature. How is he modifying animals and plants? What is 
his influence in spreading animals and plants ? In destroying them ? 

Man and the Forest. — What is the effect of the forest covering in pro- 
tecting the soil? How does it influence the distribution of rainfall? 
How does it affect the streams? State briefly the importance of the 
forest. What reasons are there for thinking that it affects the climate » 

Influence of Nature upon Man. — What change is taking place ? What 
differences do we find between people of different occupations ? How do 
the inhabitants of the several zones differ ? What was the former condi- 
tion of man? How did the surroundings influence the Chinese? The 
Egyptians? The inhabitants of the Italian peninsula? Of Greece? 
Why was the Mediterranean the natural seat of early navigation ? How 
were the Northmen influenced by surroundings ? The English? What 
are the reasons for the large number of European nations? What is 
illustrated by Switzerland? Why is America so different from Europe 
in respect to political divisions? What physical features aided in the 
discovery of America? What influence did this discovery exert? Why 
were the American settlements made near the coast? What was the 
influence of the forest barrier? Why was the settlement of the interior 



476 PHYSICAL GEOGRAPHY. 

delayed? When reached, why was its settlement relatively easy? What 
caused the development of the far west? What has determined the 
position of the towns of New England? What relation is there between 
the industries of the country and the surroundings? 

CHAPTER XXIII. 
Economic Products of the Earth. Pages 420-430. 

Soil. — What is its origin? Its value? 

Building Stones. — What is the origin of granite? What other stones 
are sold as granite? What are the metamorphic building stones? What 
is the origin of slate? Of marble? What are the causes of metamorph- 
ism? What are the sedimentary building stones? How abundant are 
they? What other mineral substances are used for building? What is 
the origin of the clay deposits ? 

Economic Deposits of Sedimentary Origin. — What is the origin of the 
substances used for grinding and polishing? Of rock salt? What other 
substances occur with it ? What is the origin of the fertilizers? 

Miscellaneous Substances. — What are some of these ? 

Coal. — What evidence is there pointing to the origin of coal? How 
may coal have been formed by drifting wood? By accumulation in bogs? 
On seashore marshes ? What is the probable origin of coal ? How is the 
coal changed? What are these changes? In what periods in the earth's 
history has coal been formed ? 

Natural Gas and Petroleum. — What is their value ? How do they 
occur? How constant is their supply? What is their origin? What 
artificial products do they resemble ? 

Ore Deposits. — In what associations do metals occur ? How must they 
occur to be profitable? Describe replacement deposits. Fissure deposits. 
Sedimentary deposits. What are placer deposits ? Where do they occur ? 
What other substances, besides gold, occur in this way? 

Distribution of Ore Deposits. — Where do they most commonly occur? 
Why are so few of the metals produced from the Cordilleras ? What are 
the reasons for the importance of the Cordilleras ? 

Mineral Wealth of the United States. — In the production of what 
metals does this country take first rank? In what does it take second 
rank ? How valuable is the industry, and how is it distributed ? Which 
is the leading state ? Its products? The second? Its products? The 
third ? The fourth ? What has been the value of this great wealth ? 



INDEX. 



Absolute humidity, 37, 434. 
Absorption of heat, 30 ; of light. 28 ; of 

vapor, 36. 
Accidental winds, 70, 82. 
Accidents to river valleys, 275. 
Active volcanoes, 377. 
Adirondack^, 256, 304, 409, 410 ; lakes in, 

299, 300 ; peaks of, 356. 
Adjustment of streams, 272. 
Aerial life, 135. 
Age of earth, 218, 221. 
Ages, geological, 220. 
Air, effect of heat upon, 68. 
Air currents, deflection of, by rotation, 

39. 
Alaska, glaciers in, 308, 311, 312, 313. 
Algeria, high temperature of, 63. 
Alkaline plains, 394. 
Alluvial fan, 285, 288. 
Alpine glacier, 307. 
Alpine snow field, 306. 
Alps, 368; glaciers in, 308; valleys in, 

271, 272. 
Altitude, effect upon temperature, 47. 
American Falls, Niagara, 295. 
Andromeda nebula, 17. 
Anemometer, 433. 
Aneroid barometer, 433. 
Animals, aid in disintegrating rocks, 

235; effect on coast, 340; habits of, 

141, 142 ; importance in ocean, 395 ; of 

ocean bottom, 156, 169, 171. 
Antarctic, icebergs in, 316 ; ice sheet of, 

313. 
Antecedent valleys, 278, 365. 
Anticline, 208, 209. 
Anticyclones, 100. 
Anti-trade winds, 70, 74. 



Apogee, 13; effect of, upon tide, 200. 

Appalachian Mountains, 255, 368. 

Arctic climate, 132 

Arctic, life in, 138. 

Arctic weather, 125. 

Argon in atmosphere, 24. 

Arid land drainage, 280 ; vegetation, 141, 
142. 

Arroya, 279. 

Artemesia geyser, 387. 

Artesian wells, 229. 

Ash, volcanic, 371, 373. 

Asia, monsoons of, 77. 

Asteroids, 6, 11. 

Atlantic, 249; circulation of, 72, 73; 
coast plains, 254 ; cross-section of, 
158, 251 ; temperature of, 181 ; tides^ 
of, 194 ; topography of bottom, 
158 ; volcanoes in, 370 ; windj of, 72, 
73. 

Atmosphere, 5 ; absorption of vapor by, 
36 ; circulation of, 68 ; composition of, 
23 ; cooling of, on ascension, 33 ; den- 
sity of, 23, 24 ; effect of earth's rota- 
tion on, 39; effect of gravity on, 39; 
effect of heat upon, 68; extent of, 23; 
moisture in, 35; pressure of, 39; 
saturation of, 36 ; warming of, 32, 33. 

Atmospheric circulation, parts of, 69. 

Atmospheric electricity, 29. 

Atmospheric movements, effect of, upon 
temperature, 44. 

Atolls, 342. 

Aurora, 29. 

Australia, animals of, 145 ; monsoons 
of, 77. 

Avalanche blast, 70, 82. 

Avalanches, effect upon rivers, 282 ; for- 
mation of, 241. 

Avalanche lake, N.Y., 299. 



477 



478 



PHYSICAL GEOGRAPHY. 



B. 



Bad Lands, S.D., 247. 

Baker's Park, 357. 

Bank of river, 262. 

Banner cloud, 111. 

Barograph, 433. 

Barometer, 433 ; change during passage 
of hurricane, 86. 

Barometric gradient, 

Barrier reefs, 341. 

Bars, 331, 334, 335, 394, 395; in rivers, 
288. 

Base level, 265. 

Basin of Minas, tidal flat in, 202. 

Basin Ranges, 258. 

Bay of Fundy, tides of, 196, 197. 

Bays, origin of, 276, 277, 329. 

Beaches, 335, 336, 395 ; abandoned, 349. 

Bermudas, depth of ocean near, 158. 

Black-bulb thermometer, 432. 

Blizzards, 100. 

Bonneville, Lake, 302. 

Borax, 422. 

Bosses, 212, 383. 

Boulder clay, 321. 

Boulders in moraine, 320, 321 ; on sea- 
coast, 336. 

Breakers on the coast, 174, 175. 

Brines, 422. 

British Isles, tides near, 194, 195. 

Bromine, 422. 

Building stone, 420. 

Butte, 353, 356, 383, 402. 

Buzzard's Bay, tides of, 197. 



Calm belts, migration of, 70, 76. 

Campos, 122. 

Canadian Highlands, 256. 

Canons, 240, 242, 267, 270. 

Canon of Colorado, 270, 352, 391. 

Cape Ann, Mass., coast of, 203, 334-338, 
400, 401 ; moraine of, 320 ; salt marsh 
on, 339 ; sand dunes on, 238. 

Cape Cod, Mass., sea cliff, 328. 

Capes, origin of, 329, 345, 346. 

Carbonic acid gas in atmosphere, 24. 

Casco Bay, Me., islands of, 345. 



Cave, 227 ; river source in, 263. 

Caverns, formation of, 226. 

Cementing of rocks, 217. 

Centigrade scale, 431. 

Central plains, 256. 

Ceres, 11. 

Charleston earthquake, 384. 

Chasms, origin of, 400. 

Chemical deposits from underground 
water, 225. 

Cherrapunji, rainfall of, 122. 

Chesapeake Bay, origin of, 278. 

Chicago, lake breeze of, 80. 

Chili, changes of level in, 206. 

China, loess in, 399. 

Chinook wind, 99. 

Chromosphere, 7. 

Chronology, geological, 218. 

Circulation of atmosphere, 68 ; of ocean, 
182; of water on ocean bottom, 163, 
172. 

Circumpolar whirl, 75, 101. 

Cirro-cumulus cloud, 113. 

Cirro-stratus cloud, 113. 

Cirrus cloud, 113. 

Clays, 421. 

Climate, 129; of arctic zone, 132; 
changes in, 132, 317 ; effect upon lakes, 
302 ; effect on man, 413 ; effect of upon 
streams, 267, 279; influence upon 
topography, 398 ; minor variations of, 
132; of plateaus, 351 ; of St. Louis, 62; 
of San Francisco, 62; study of, 435; 
of temperate latitude, 130; of trop- 
ical regions, 130. 

Climatic zones, 129. 

Cloudbursts, 104, 123. 

Clouds, 111; kinds of, 112; study of, 
434; of cyclonic storms, 94. 

Coal, 423. 

Coast, cause of irregularities of, 330; 
changes in, 343 ; destruction of, 332 ; 
effect of tides on, 201; effect of waves 
upon, 176. 

Coast line, 328, 395 ; of United States, 
261. 

Coast Ranges, 259. 

Coastal plain, 254, 393. 

Cold pole, 56. 

Cold waves, 51, 100, 127, 128. 



INDEX. 



479 



Color, cause of, 29 ; of ocean water, 152. 

Colorado canon, 270, 352, 391 ; rapids in, 
298. 

Colorado, mineral wealth of, 430. 

Comets, 6, 15. 

Complex valleys, 276. 

Composite valleys, 276. 

Concretions, 427. 

Conduction of heat, 32. 

Cone delta, 285. 

Cone of volcano, 378. 

Consequent river courses, 272. 

Constructional land forms, 392. 

Continental glacier, 307, 313, 318. 

Continental islands, 344. 

Continental shelf, 158. 

Continental slope, 159. 

Continents, 249 ; cause of, 390 ; change 
in form of, 252 ; features of, 251. 

Contorted limestone, 214. 

Contour interval, 439. 

Contour maps, 438. 

Contraction theory, 363. 

Convection, 32. 

Coral deposits, 395. 

Coral islands, 395. 

Coral keys, 341. 

Coral reefs, 168, 341. 

Corals, conditions favoring develop- 
ment of, 169 ; effect of Gulf Stream 
upon, 191; effect of ocean currents 
on, 342 ; effect of temperature on, 
136 ; importance on coast, 340. 

Cordillera, 354. 

Cordilleras, age of, 259 ; minerals of, 
259 ; ores of, 428, 429 ; volcanoes of, 
371 ; of the west, 257. 

Corona, 7, 28. 

Crater of geysers, 388 ; of volcanoes, 
379. 

Crevasse, 309. 

Crust of earth, 205 ; movements of, 206, 
390 ; rocks of, 212. 

Crystalline rocks, 213. 

Cumberland valley, model of, 437. 

Cumulo-stratus clouds, 113, 114. 

Cumulus cloud, 113. 

Currents of ocean, 163, 172, 182, 328; 
deflection of, by earth's rotation, 39 ; 
effect of, on coast, 330, 332. 



Cyclonic storms, 85. 

D. 

Daily temperature ranges, 43, 59, 60 
61, 65. 

Day, cause of, 13. 

Dead Sea, life in, 137. 

Death Valley, 258. 

Deccan, plateau of, 351. 

Deep-sea animals, oxygen supply of, 171. 

Deep sea, circulation of water in, 163, 
172 ; dredging of, 155 ; exploration of, 
153; life in, 169; light in, 163; sedi- 
ments of, 164; sounding of, 154; 
sounding machine, 154; temperature 
of, 162 ; trawl, 154, 155. 

Deforesting Adirondacks, 410. 

Delaware Bay, origin of, 277. 

Delta lakes, 300. 

Delta of Mississippi, 344. 

Deltas, 285, 331, 394 ; conditions favoring 
formation of, 286 ; in lakes, 287 ; rela- 
tion to floodplain, 288 ; rivers on, 287. 

Denudation, 246, 390, 393, 395-397 ; ab- 
sence of effects of, on ocean floor, 156 ; 
of land, 224 ; of mountains, 360, 361, 
364, 367 ; of volcanoes, 379. 

Depression, effect on coast, 329. 

Depth of ocean, 157, 158. 

Desert dust whirl, 68. 

Deserts, cause of, 74, 117 ; life in, 141. 

Dew, 107. 

Dew point, 37. 

Diathermanous bodies, 30. 

Diffusion of light, 26. 

Dip of rocks, 209; relation of topog- 
raphy to, 402. ^ 

Dismal Swamp, 304, 425. u^"^ 

Dissection of valleys, 278. 

Distributaries on deltas, 287. 

Diurnal winds, 70, 76, 79. 

Diversion of streams by mountain 
growth, 278. 

Divide, 263 ; changes in, 273. 

Doldrums, climate of, 130; density of 
ocean in, 152 ; migration of, 70, 76, 122 ; 
rains of, 117 ; thunderstorms of, 102 ; 
weather of, 124. 

Donati's comet, 15. 



480 



PHYSICAL GEOGRAPHY. 



Dormant volcanoes, 377. 

Drainage of mountains, 365. 

Dredging, 155. 

Droughts, cause of, 126. 

Drowned rivers, 276, 277. 

Dust, effect of, on light, 26, 27 ; in at- 
mosphere, 24; importance in forma- 
tion of fog, 110. 

Dust whirl of the desert, 68. 

Dykes, 212, 383. 



E. 



Earth, 11; age of, 218, 221; condition 
of, 11; elevations on the surface of, 
4; form of, 3; interior condition of, 
11, 205 ; irregularities on surface, 3 ; 
movements of, 12, 33; movements 
of surface, 206; revolution of, 12; 
rotation of, 13 ; water on the surface 
of, 4. 

Earth columns, 232. 

Earth temperature, 65. 

Earthquake waves, 178. 

Earthquakes, 383 ; association with vol- 
canoes, 381 ; cause of, 385. 

Eastern mountains, 254. 

Eastport, Me., tides at, 199, 200. 

Ebb of tide, 192. 

Eclipse breezes, 70, 76, 82. 

Electricity, 29. 

Elevation, effect on coast, 329. 

Elk Mountains, Col., 354. 

English Channel, tides of, 194, 195. 

Epicentrum of earthquakes, 384. 

Erosion, agents of, 238 ; by glaciers, 245 ; 
by oceanic forces, 244, 328; by rain, 
239 ; by rivers, 241, 265, 268 ; by under- 
ground water, 240 ; of volcanoes, 379 ; 
by wind, 238. 

Eruption of geysers, 389. 

Estuary, filling with salt marsh, 339. 

Estuaries, origin of, 276, 277, 329. 

Eurasia, map of, 250. 

Europe, glaciation of, 318. 

Evaporation of water, 31, 35, 39, 120; 
measurement of, 434. 

Extinct lakes, 302; volcanoes, 378, 
381. 

Eye of storm, 87, 96. 



P. 



Fahrenheit scale, 431. 

Fall line, 296. 

Fan delta, 285. 

Fathom, 154. 

Fault, 210../-" 

Fault plane, 210. 

Faults, association of earthquakes with, 

386 ; relation of ores to, 428. /■ 
Faunas of ocean bottom, 169 ; of ocean 

surface, 166. 
Fertilizers, 422. 
Finger Lakes, origin of, 325. 
Floe ice, 315. 
Floodplains, 288, 394; characteristics 

of, 291 ; building of, 293. 
Floods, influence of forest upon, 410. 
Florida, growth of, 347; keys of, 341; 

lakes of, 300; swamps of, 303, 424. 
Flow of tide, 192. 
Focus of earthquakes, 384. 
Foehn wind, 99. 
Fog, 109. 

Food of ocean animals, 168. 
Food supply, effect on ocean life, 171. 
Forest in Adirondacks, 409. 
Forest, importance of, 409 ; influence on 

development of United States, 417; 

influence of man upon, 409 ; on moun- 
tains, 360. 
Forest litter, 410. 
Fossils, value of, 219. 
Fresh water life, 137. 
Frost, 108; action on mountain peaks, 

361 ; aid in disintegrating rocks, 234. 
Fusiyama, Japan, 380. 



G. 



Ganges delta, effect of hurricane on, 89. 

Garden of Gods, Col., 231. 

Gas, 425. 

Gassendi, lunar crater of, 14. 

Gay Head, retreat of, 333. 

Geographic distribution of animals and 

plants, 135. 
Geological ages, 220. 
Geological chronology, 218. 
Geysers, 386, 387. 



INDEX. 



481 



Glacial deposits, 394 ; formation of lakes 
by, 299 ; production of waterfalls by, 

294 - ^ 

Glacial erosion, 245. 

Glacial lakes, 299, 317, 323. 

Glacial period, 133, 316 ; effect upon life, 

146 ; effect upon streams, 280 ; time of, 

319. 
Glacial scratches, 322. 
Glacial soil, 321. 
Glaciers, Alpine, 307; in Antarctic, 313, 

316; cause of, 122, 306; continental, 

307, 313, 318 ; effect upon valleys, 280 ; 

in Greenland, 313, 314 ; Piedmont, 313 ; 

relation to swamps, 304. 
Globigerina ooze, 164; area of deposit, 

166. 
Gneisses, 420. 
Gold deposits, 428. 
Gorges, caused by glacial action, 281 ; 

formation of, 242, 325 ; in mountains, 

271, 272, 358, 365 ; near Ithaca, N.Y., 

215, 265. 
Graham's Island, 346. 
Granite, 420 ; disintegration of, 233. 
Gravity, aid in erosion, 240; effect on 

atmosphere, 39. 
Great Barrier Reef, Australia, 341. 
Great Basin, 258, 281, 357 ; drainage of, 

279; mountains of, 258; temperature 

of, 56. 
Great Lakes, effect of ice on, 281 ; origin 

of, 325 ; winds of, 80. 
Great Salt Lake, 303 ; former extension 

of, 281. 
Greenland, glaciers of, 313, 314; winds 

of, 78. 
Green River, Utah, 278. 
Ground moraine, 312. 
Gulf of Mexico, temperature of bottom, 

163. 
Gulf Stream, 183, 187; effect on corals, 

191, 347 ; effect on life, 167, 168 ; effect 

on temperature, 51, 55, 190; map of, 

188 ; velocity of, 187. 
Gulf weed, 136. 
Gypsum, 422. 



Hachure maps, 437, 438. 



Hail, 116. 

Halo, 28. 

Harbors, sea action in, 334. 

Hawaiian Islands, volcanoes of, 377, 
380. 

Haze, 110. 

Heat, 30 ; absorption of, 30 ; distribution 
of, 43; effect of movements of the 
earth upon, 33 ; effect upon air, 68. 

Heat equator, 53, 55. 

Heat lightning, 30. 

Heligoland, destruction of, 332. 

Hell Gate, tide at, 196, 198. 

Hercuianeum, destruction of, 376. 

High pressure, 433. 

Hills of circumdenudation, 356. 

Himalayas, 110, 368. 

Hooks, 333, 334, 347. 

Horse latitude winds, 70, 75. 

Hot springs, 386. 

Humidity, absolute, 37; relative, 37; 
measurement of, 434; variation in, 38. 

Hurricane, 86 ; cause of, 91 ; cause of 
path of, 93; destruction caused by, 
89 ; difference from temperate latitude 
cyclones, 95; effects of, 88; features 
of, 87 ; importance of vapor in, 92 ; 
paths of, 89, 90, 97 ; pressure in, 86, 88 ; 
reason for absence from South Atlan- 
tic, 92 ; reason for development over 
ocean, 92; resemblance to temperate 
latitude cyclones, 93 ; size of, 90 ; time 
of occurrence of, 91; violence of, 90; 
winds of, 87, 88. 

Hygrometer, 434. 

I. 

Icebergs, 314-316. 

Ice cave, 312. 

Ice fall, 309. 

Igneous rocks, 213, 420 ; relation of ores 
to, 428. 

India, monsoon of, 77. 

Indianola, Tex., destruction by hurri- 
cane, 89. 

Interior basins, 356. 

Intruded rocks, 212, 213. 

Irregular winds, 70, 82. 

Island life, 145. 



2i 



482 



PHYSICAL GEOGRAPHY. 



Islands, destruction of, 346; origin of, 

329, 344 ; volcanic, 244. 
Isothermal charts, 51. 
Isotherms, 51 ; of New York, 56, 59 ; of 

United States, 54, 56-58; relation to 

climate, 62. 
Ithaca, N.Y., change in harometer at, 

86 ; cold wave at, 127, 128 ; gorges near, 

265, 297; humidity changes in, 37; 

temperature changes in, 61, 66; valley 

breeze at, 81 ; waterfall near, 297. 



Japan, earthquake in, 385-387. 
Japanese current, 184 ; effect on temper- 
ature, 190. 
Jupiter, 9. 

K. 

Key West, temperature of, 53, 56, 63, 65. 

Keys, 341. 

Krakatoa, eruption of, 374, 375, 380, 381, 

385. 
Kurile Islands, depth of ocean near, 160. 
Kuro Siwo, 184. 



L. 



Labrador current, 189 ; effect upon tem- 
perature, 53, 55, 168. 

Lagoon, 348. 

Lake Agassiz, 324. 

Lake Bonneville, 302. 

Lake breeze, 80. 

Lake Champlain, origin of, 325. 

Lake Drummond, origin of, 300. 

Lake Erie, destruction of, by Niagara, 
301. 

Lake spit, 333. 

Lakes, 298, 394; caused by beach bar- 
riers, 335, 348; caused by lava, 374, 
381 ; deltas in, 287 ; destruction of, 
300 ; extinct, 302 ; on floodplains, 292 ; 
glacial, 281, 317, 323 ; in Adirondacks, 
409 ; in mountains, 366, 367 ; in young 
valleys, 269 ; relation to swamps, 303 ; 
shores of, 328, 348, 394. 

Land breeze, 70, 79. 



Land, denudation of, 224 ; effect on tem- 
perature, 55 ; effect on tide, 193 ; ele- 
vation of, 206; life, 135, 137; move- 
ment, effect on coast, 329; topography 
of, 390. 

Land-locked animals, 137. 

Landslide, formation of, 241. 

Landslip blast, 70, 82. 

Latent heat, 31, 35, 39. 

Lateral moraine, 310, 312. 

Lava, 371. 

Lava flow, 211, 372. 

Lava plateaus, 351. 

Lawrence, Mass., tornado, 105. 

Levees, 293. 

Life, barriers to the spread of, 146 ; de- 
struction by volcanic eruption, 381; 
effect of man upon, 146, 147 ; effect of 
ocean currents on, 191 ; of the air, 135 ; 
of the arctic zones, 138 ; of the dead 
seas, 137 ; of the deserts, 141 ; of the 
fresh water, 137; of the land, 135, 
137; of the mountains, 140; of the 
ocean, 135, 166; of the ocean bottom, 
153, 156, 169; of the ocean bottom, 
oxygen supply of, 171 ; of the ocean 
shore, 167 ; of the temperate zones, 
139; spread of, 145. 

Life zones, 135, 143; of United States, 
144. 

Light, 25 ; absorption of, 28 ; diffusion 
of, 26; effect of dust on, 26, 27; on 
ocean bottom, 163; reflection of, 27; 
refraction of, 27 ; selective scattering 
of, 26; source of, 25. 

Lightning in thunderstorms, 29, 102, 
104. 

Limestone, 421. 

Line storm, 91. 

Lipari Islands, volcanoes of, 375. 

Littoral faunas, 167. 

Llanos, 122. 

Loess in China, 399. 

Longitudinal valleys in mountains, 358, 
365. 

Long's Peak, Col., 360. 

Looming, 27. 

Low pressure, 433. 

Low-pressure areas, tracks of, 95-97. 

Lunar craters, 15. 



INDEX. 



483 



M. 

Magnetic pole, 29. 

Magnetism, 29. 

Malaspina glacier, 313. 

Mammoth hot springs, 225. 

Man and nature, 407. 

Man and the forest, 409. 

Man, effect in distributing life, 146, 147 ; 

modifying influence of, 407. 
Mangrove, 338. 
Mangrove swamps, 424. 
Marble, 421. 
Mars, 9. 

Marsh grass, 339. 
Massachusetts, lakes in, 324. 
Massachusetts Bay, tides of, 197. 
Mato Tepee, Wyo., 383. 
^^Matterhorn, 355. 

Mature adjustment of streams, 273. 
Mature river valleys, 266, 267. 
Maximum temperature in United States, 

64. 
Maximum thermometer, 432. 
Mechanical sediments in ocean, 164. 
Medial moraine, 310, 312. 
Mediterranean, temperature of water 

in, 162, 163; tides of, 197. 
Mercury, 8. 
Mesas, 353, 383. 

Metals, 426 ; of Cordilleras, 259. 
Metamorphic rocks, 213, 214, 420. 
Meteorites, 16. 
Meteors, 6, 15, 16. 
Michigan, mineral wealth of, 429. 
Mid- Atlantic ridge, 159. 
Mineral waters, 423. 
Minerals, 213, 426; of the Cordilleras, 

259; disintegration of, 233; effect of 

water upon, 226 ; of United States, 

429. 
Minimum temperatures in United States, 

63. 
Minimum thermometer, 432. 
Minnesota, lakes in, 324. 
Mirage, 27. 
Mississippi, delta of, 286, 344 ; floodplain 

of, 290, 291, 292. 
Mississippi valley plains, 256. 
Mist, 111. 



Mitchell's Peak, height of, 256. 

Models, 437. 

Moisture, effect upon life, 141; in the 
atmosphere, 35 ; measurement of, 434. 

Monocline, 208. 

Monoclinal shifting, 274. 

Monsoon winds, 70, 77 ; effect upon cli- 
mate, 130 ; effect upon rainfall, 122. 

Montana, mineral wealth of, 429 ; tem- 
perature changes in, 56, 64, 65. 

Monte Somma, 376, 380. 

Moon, 13 ; effect in producing tide, 192, 
199-201. 

Moqui Pueblo, N.M., 239. 

Moraines, 310, 312, 317, 319, 394. 

Mount Dana, glaciers on, 310. 

Mount Desert, Me., 411; coast of, 330. 

Mount Everest, 110. 

Mount Hood, 378. 

Mount of Holy Cross, Col., 359. 

Mount Marcy, height of, 256. 

Mount St. Elias, 139 ; glaciers of, 312. 

Mount Shasta, 382 ; glaciers on, 307. 

Mountain breeze, 70, 80. 

Mountain gorges, 358. 

Mountain thunderstorms, 102. 

Mountain valleys, 271, 356. 

Mountain vegetation, 140. 

Mountains, association of volcanoes 
with, 370; association with plateaus, 
350 ; cause of, 390 ; characteristics of, 
353 ; in continents, 251 ; denudation 
of, 396, 404 ; destruction of, 367 ; drain- 
age of, 365 ; of eastern United States, 
254 ; effect of growth of, upon streams, 
278; effect upon temperature, 48; 
floodplains among, 289; glaciers in, 
307; of Great Basin, 258; growth of, 
363, 367; life in, 140; origin of, 362, 
393; ruggedness of, 361, 396; sculp- 
turing of, 364 ; valleys in, 262-265 ; of 
the west, 257. 

Mud flow, 372. 

Muir's Butte, Cal., 379. 

N. 

Natural bridge, origin of, 228. 
Natural gas, 425. 
Natural soda, 422. 



484 



PHYSICAL GEOGRAPHY. 



Nature and man, 407- 

Nature, influence upon man, 412. 

Navajo Church, Arizona, 397. 

Neap tides, 200. 

Nebulse, 17, 21. 

Nebular hypothesis,19; verification of ,20. 

Neptune, 10. 

New York, isotherms of, 56, 59 ; tempera- 
ture of, 51. 

New York harbor, tides of, 194. 

New Zealand, animals of, 145. 

Niagara, effect in draining Lake Erie, 301. 

Niagara Falls, 264, 295, 301 ; age of, 222 ; 
history of, 294 ; origin of, 298. 

Night, cause of, 13. 

Nimbus clouds, 113, 114. 

Nitrogen in atmosphere, 23. 

North America, cross-section of, 251; 
glacier of, 318; shore line of, 261. 

North Atlantic drift, 183. 

Northeast storms, 94. 

Norther, 100. 

Nunatak, 314. 



O. 



Oblong geyser, 388. 

Occupation, relation to topography, 418. 

Ocean, area of, 4, 151; deposits in, 395; 
depth of, 160, 161 ; effect in checking 
spread of life, 146 ; effect of, on tem- 
perature, 45; erosion in, 244, 328; 
phosphorescence in, 152, 164; shores 
of, 328; surface temperature of, 179; 
volume of, 4 ; volcanoes in, 370. 

Ocean basins, 249. 

Ocean bottom, circulation of water on, 
163, 172 ; dredging of, 155 ; exploration 
of, 153, 156; life on, 153, 166, 169; 
light on, 163 ; sediments of, 164 ; tem- 
perature of, 155, 162 ; topography of, 
156, 160, 250. 

Ocean currents, 182; cause of, 185; 
cause of course, 187; effects of, 189; 
effect on life, 146, 166 ; effect on tem- 
perature, 46, 180; on ocean bottom, 
163, 172 ; system of, 183. 

Ocean water, color of, 152; composi- 
tion of, 151 ; density of, 152. ' 

Oceanic islands, 244, 344. 



Oceanic life, 135, 166; habits of, 169; 

influence of temperature upon, 136. 
Oceanic plateau, 157, 159. 
Oil, 425. 

Old Faithful Geyser, 389. 
Opaque bodies, 29. 
Ore deposits, 426. 

Oxbow cut-off lakes, 266, 292, 300. 
Oxygen, in atmosphere, 23 ; supply of, 

to deep-sea animals, 171. 



Pacific Ocean, 249; topography of bot- 
tom, 160; volcanoes in, 370. 

Parks in mountains, 357, 358. 

Passes in mountains, 359. 

Path of storms, 89, 94-97. 

Peaks in mountains, 355. 

Peaks, origin of, 404. 

Peat bogs, 304, 425. 

Pecos River valley, N.M., plain of, 350. 

Pelagic faunas, 166. 

Pennsylvania, mineral wealth of, 429. 

Percolating water, importance of, 240. 

Perigee, 14 ; effect upon tide, 200. 

Periodical winds, 70, 76. 

Permanent winds, 70, 71. 

Petroleum, 425. 

Phosphates, 422. 

Phosphorescence in ocean, 152, 164. 

Photosphere, 7. 

Piedmont glacier, 313. 

Pike's Peak, 355. 

Placer deposits, 428. 

Plains, 350; of Atlantic coast, 254; in 
continents, 251 ; of Far West, 351 ; of 
Mississippi valley, 256 ; origin of, 393 ; 
of Red River valley, 394. 

Planetary circulation in ocean, 182. 

Planetary winds, 70, 71. 

Planets, 6, 8 ; relative distance of, 5, 8 ; 
relative size of, 9. 

Plants, aid in disintegrating rocks, 234; 
effect of, on coast, 337 ; habits of, 141 ; 
in the ocean, 136, 395. 

Plateau, 350; association with moun- 
tains, 350; of continents, 251; of ice, 
314; of Mississippi valley, 256; of 
ocean bottom, 157, 159, 250. 



INDEX. 



485 



Platinum, 428. 

Pompeii, destruction of, 372, 376. 

Porto Rico, depth of ocean near, 157, 

160. 
Prairie soil, 323. 
Prairies, 257, 351, 394. 
Pressure of atmosphere, 39. 
Pressure in hurricane, 88. 
Pressure, measurement of, 432; relation 

to winds, 70. 
Prevailing westerlies, 70, 75, 
Promontories, origin of, 329, 345, 346. 
Psychrometer, 434. 
Pulpit terrace, 225. 
Pumice, 211, 371. 



R. 



Radiant energy, 30 ; effect upon water, 
31 ; effect upon the land, 31 ; passage 
through the atmosphere, 31 ; reflection 
of, 30. 

Radiation from the earth, 32. 

Rafe's Chasm, 400. 

Rain, cause of, 114. 

Rain erosion, 239. 

Rain gauge, 435. 

Rain in thunderstorm, 104. 

Rainbow, cause of, 28. 

Rainfall, distribution of, 117 ; in dol- 
drum belt, 74 ; effect of forest on, 412 ; 
irregularities of, 123; measurement 
of, 435 ; seasonal distribution of, 122 ; 
in trade-wind belt, 74 ; of the United 
States, 118. 

Ranges of mountains, 354. 

Rapids, relation to waterfalls, 294. 

Ray Brook, Adirondack^, 304. 

Red clay, 165. 

Red River valley, effect of ice on, 281 ; 
lake in, 324 ; plains of, 350, 394. 

Red Sea, cause of color of, 152. 

Reefs, coral, 341. 

Reflection of radiant energy, 30. 

Refraction of light, 27. 

Rejuvenation of river valleys, 276. 

Relative humidity, 37, 434. 

Replacement deposit, 427. 

Residual soil, 238. 

Revived rivers, 276. 



Revolution, effect of, upon temperature, 
33. 

Rhone glacier, 308. 

Ridges, mountain, 354, 361, 368, 405. 

Right-hand deflection, 40. 

Rio Grande valley canon, 142; talus in, 
236. 

River bank, 262. 

Rivers, boulders in bed of, 243; acci- 
dents to, 275; characteristics of, 263; 
deposits by, 394; divide of, 273; effect 
of forest on, 410; erosion of, 241, 243; 
on floodplains, 291 ; at margin of ice, 
312, 322 ; in mountains, 365 ; relation 
of lakes to, 299 ; sediment in, 241 ; of 
United States, 259, 260. 

River system, 263. 

River valleys, 262 ; adjustment of, 272 ; 
drowned by sea, 330 ; development of, 
265; difference in rate of develop- 
ment of, 270; effect of climate on, 
279 ; origin of, 264 ; variation among, 
244. 

Rock basins, 325. 

Rock pillars, 231. 

Rock salt, 422. 

Rocks, consolidation of sedimentary, 
217; deposition of sedimentary, 215; 
disintegration of, 233; disturbance of, 
207; durability of, 231; of earth's 
crust, 212 ; elevation of, 216 ; horizon- 
tal, 208 ; igneous, 213 ; influence of, on 
form of crust, 334; influence upon 
stream course, 272; influence upon 
topography, 208, 395, 402-405; in- 
truded, 212, 213; metamorphic, 213, 
214; of mountains, 355, 362; sedimen- 
tary, 213, 214. 

Rocky Mountains, 257, 368. 

Rotation, deflective effect of, 39 ; effect 
of, on temperature, 33. 

Royal Gorge, Col., 265. 



S. 



St. Anthony, Falls of, 296. 
St. Louis, temperature of, 62. 
Salt lakes, 302. 
Salt marsh, 332, 339. 
Salts in the ocean, 151. 



488 



PHYSICAL GEOGRAPHY. 



Samoan Islands, hurricane of, 88. 

San Francisco, temperature of, 62. 

Sand bars, 331. 

Sand dunes, 239, 394. 

Sands, 421. 

Sandstone, 421. 

Sargasso Sea, 136, 167. 

Satellites, 6. 

Saturation of atmosphere, 36. 

Saturn, 10. 

Sea breeze, 45, 70, 79. 

Sea caves, 334, 335, 400. 

Sea cliffs, 347, 400, 401, 403; Cape Cod, 
Mass., 328; retreat of, 332. 

Seasonal temperature range, 43, 48, 49, 
51. 

Seasonal winds, 70, 76. 

Seasons, 12, 13, 33. 

Seaweeds, importance on coast, 337, 338. 

Secondary storms, 101. 

Sediment, effect of, on coast, 330 ; on 
ocean bottom, 164 ; in rivers, 241. 

Sedimentary rocks, 213, 214, 330, 421 ; 
consolidation of, 217 ; deposition of, 215. 

Seeds, aid in distribution of plants, 141. 

Seiches, 198. 

Selective scattering, 26. 

Shastina, 382. 

Shooting stars, 15, 16. 

Shore faunas, 167. 

Shore lines, 328, 395 ; above sea level, 
207 ; change in, 343, 400 ; effect of tide 
on, 201 ; fossil, 349 ; of lakes, 348 ; of 
United States, 261. 

Shrunken streams, 279. 

Siberia, low temperature of, 56, 63. 

Sierra Nevada Mountains, 258. 

Signal Butte, 402. 

Sigsbee deep-sea sounding machine, 154. 

Silver deposits, 428. 

Sink-holes, 226. 

Sirocco wind, 99. 

Slate, 421. 

Small planets, 11. 

Snake River valley, lava plateau of, 351, 
373. 

Snow, 115. 

Snowfall, distribution of, 121 ; measure- 
ment of, 435. 

Snow field, 306, 308. 



Snowflakes, 115. 

Snow line, 139, 140. 

Soil, 420 ; effect of forest on, 441 ; for- 
mation of, 237 ; glacial, 321. 

Solar light, 25. 

Solar system, 5; symmetry of, 18. 

Sounding, 153. 

Sphagnum moss, 304. 

Spits, 333, 334, 347, 394. 

Spring tide, 199. 

Springs, effect of forest on, 410 ; origin 
of, 228. 

Stalactites, 227. 

Stalagmites, 227. 

Stars, 17, 18. 

Steam in volcanoes, 372, 383. 

Stellar system, 17. 

Storms, 85 ; conditions in, 88, 94 ; of 
secondary origin, 101 ; tracks of, 89, 
94-97 ; waves accompanying, 177, 
179 ; winds of, 70, 82, 85, 94, 98. 

Straits, origin of, 276, 277. 

Strata, 216 ; influence on topography, 
401-405 ; in mountains, 364. 

Stratification, 216. 

Stratified rocks, 215. 

Stratus clouds, 113, 114. 

Stream gold, 428. 

Strike, 209. 

Summer, temperature of, 50. 

Sun, 6; effect in producing tide, 193, 
199, 201 ; movements of, 8. 

Sun spots, 8. 

Sunset colors, 26. 

Surface faunas in ocean, 166. 

Swamps, 303, 394 ; of Florida, 424, 425 ; 
of glacial origin, 281, 283 ; mangrove, 
339. 

Sweden, changes of level in, 206. 

Syncline, 208. 

Synclinal mountains, 369. 

System of mountains, 354. 

T. 

Talus, 236, 240, 354. 
Taughannock Falls, 294. 
Temperate climate, 130. 
Temperate latitude cyclones, 86 ; cause 
of, 100 ; cause of path of, 101 ; differ- 



INDEX. 



487 



ence from hurricanes, 95 ; effects of, 
98 ; features of, 94 ; path of, 97 ; rela- 
tion of, to thunderstorms, IG3 ; resem- 
blance to hurricanes, 93 ; size of, 96 ; 
time of occurrence of, 96; winds of, 
94,98. 

Temperate latitude, weather of, 125. 

Temperate zone, life in, 139. 

Temperature, of Atlantic, 181 ; daily 
ranges in, 59, 65; in cold wave, 127, 
128; of earth, 65, 205; effect of alti- 
tude upon, 47 ; effect of atmospheric 
movements upon, 44; effect of land 
upon, 55, 56, 57; effect upon land 
life, 137 ; effect upon mountain life, 
140; effect of mountains upon, 48; 
effect of ocean upon, 45; effect of 
ocean currents on, 46, 189 ; effect upon 
ocean life, 136; effect of sea breeze 
on, 79, 80; effect of topography upon, 
47, 56; of Great Basin, 56; of Key 
West, 53, 55, 56; maximum, in United 
States, 64; measurement of, 431; of 
midsummer, 50; of midwinter, 50; 
minimum, in United States, 63 ; ranges 
in, 61, 62, 64 ; seasonal range of, 35, 43, 
48 ; of St. Louis, 62 ; of San Francisco, 
62; of United States, 53; variation 
of, 35, 43, 51, 60, 61. 

Temperature of ocean, 180; effect on 
circulation, 182, 185; effect on life, 
166, 168, 170. 

Temperature of ocean bottom, 155, 162, 
170. 

Temperature of ocean surface, 179, 181. 

Terminal moraine, 310, 312, 319. 

Terraces, 323, 400. 

Texas, bars on coast of, 331 ; monsoons 
of, 78 ; temperature changes in, 65. 

Thermograph, 432. 

Thermometer, 431. 

Thermometer shelter, 432. 

Thibet, temperature ranges in, 65. 

Thousand Islands, origin of, 348. 

Thunder, 30, 102, 104. 

Thunderstorms, 101-103. 

Tidal action in ocean, 328. 

Tidal bore, 198. 

Tidal breezes, 70, 76, 82. 

Tidal currents, importance of, 201, 333. 



Tidal height, causes for variation in, 193- 
203. 

Tidal flat, Basin of Minas, 202. 

Tidal races, 198. 

Tidal wave, 192. 

Tide-power, uses of, 202. 

Tides, cause of, 192; effects of, 201; 
effect of coast upon, 193-198; in Eng- 
lish Channel, 194, 195 ; in New York 
harbor, 194. 

Till, 321, 394. 

Timber line in mountains, 138, 140, 359, 
360. 

Tin, 428. 

Topographic maps, 437. 

Topography, influence upon climate, 
47, 56 ; influence on man, 413^419 ; of 
bottom of Atlantic Ocean, 158; of 
glaciated regions, 320, 326 ; of the land, 
390; of ocean bottom, 156, 160, 161, 
250 ; relation to rock structure, 395. 

Tornadoes, 104. 

Trade-wind belt, 70, 71 ; climate of, 130; 
effect on oceanic circulation, 186; 
rain caused by, 117 ; weather in, 124. 

Translucent bodies, 29. 

Transparent bodies, 28. 

Transverse mountain valleys, 365. 

Trawl, deep-sea, 155. 

Tributaries of river, 263, 269 ; on flood- 
pi ains, 293. 

Tropical climate, 130. 

Tropical cyclones, 86. 

Tropical forest, 143. 

Tropical weather, 124. 

Typhoons, 86. 

U. 

Unconformity, 217. 

Underground water, 224, 233, 240, 386. 

Undulatory theory, 25. 

United States, drainage of, 259, 260; 
evaporation in, 120; ice sheet of, 
318; isotherms of, 53, 54, 56-58; life 
zones of, 144; maximum temperature 
of, 64 ; mineral wealth of, 429 ; mini- 
mum temperature in, 63: monsoon 
tendency in, 78; ores of, 428; physical 
geography of, 253; rainfall of, 118, 
119, 122; shore line of, 261; temper- 



488 



PHYSICAL GEOGRAPHY. 



ature ranges in, 62, 64 ; terminal mo- 
raine of, 320 ; volcanoes in, 259, 371. 
Uranus, 10. 



Valley breeze, 70, 80. 

Valley fog, 109. 

Valley glaciers, 307; former extension 
of, 317. 

Valley sides, 262. 

Valleys, development of, 242, 262, 267, 
270; effect of climate on, 279; effect 
of land movements on, 276 ; in moun- 
tains, 356. 

Vapor, absorption of, 36 ; importance of, 
in hurricanes, 92 ; variation in amount, 
36. 

Vegetation, in arid land, 141, 142; in 
mountains, 140 ; in swamps, 303. 

Veins, 427. 

Venus, 9. 

Vesuvius, 372, 376, 380. 

Vineyard Sound, tides of, 197. 

Volcanic action, 211. 

Volcanic ash, 211, 371, 373. 

Volcanic cone, form of, 378. 

Volcanic island, 244. 

Volcanic necks, 382, 383. 

Volcanic winds, 70, 83. 

Volcanoes, association with atolls, 343 
association of earthquakes with, 385 
association of hot springs with, 387 
association with ores, 428; cause of, 
383; destruction of, in sea, 346; dis- 
tribution of, 370 ; effect of eruptions, 
381 ; effect upon rivers, 282 ; eruptions 
of, 374 ; extinct, 378, 381 ; materials 
erupted by, 211, 371; in ocean, 156; 
origin of, 393 ; of United States, 259. 

Vulcano, 375. 

W. 

Water, area of, on earth, 151; effect 
upon rocks, 231, 233; importance in 
volcanoes, 383 ; underground, 224, 240. 

Water vapor in atmosphere, 24. 

Waterfall breeze, 70, 83. 

Waterfalls, 268, 281, 294, 297, 325. 

Water parting, 263. 

Waterspout, 106. 



Waterspout waves, 179. 

Watkins Glen, N.Y., 326. 

Waves, 174 ; action of, on coast, 176, 244, 
328, 330, 332; cause of, 176; earth- 
quake, 178, 385 ; form of, 175 ; storm, 
179. 

Weather, 124 ; arctic, 125 ; temperate 
latitude, 125 ; tropical, 124 ; study of, 
435. 

Weather maps, 435. 

Weather predictions, 435. 

Weathering, 233 ; effects of, 236 ; impor- 
tance of, 235, 265; of volcanoes, 
379. 

Westfield River, Mass., 243. 

White glacier, Alaska, 311. 

White Mountains, N.H., 356. 

Whitney glacier, 307. 

Wind vane, 433. 

Wind waves, 174. 

Winds, accidental, 70, 82 ; action of, 393 ; 
aid in causing rain, 117; aid in distri- 
bution of animals, 145 ; of Atlantic, 72, 
73 ; classification of, 70 ; in cold wave, 
127, 128; diurnal, 70, 76, 79; effect 
upon height of tide, 198 ; effect upon 
temperature, 44; erosion by, 238; in 
the general circulation, 69; of horse 
latitude belt, 75 ; of hurricane, 87, 88 ; 
internal work of, 83 ; irregular, 70, 82 ; 
irregularities of, 83; measurement of r 
433; migration of, 76; monsoon, 70, 
77; nature of, 83; periodical, 70, 76: 
permanent, 70,71; planetary, 70, 71; 
seasonal, 70, 76 ; of storm, 85, 94 ; of 
temperate latitude cyclones, 94, 98; 
of temperate latitudes, 75 ; in thunder- 
storms, 103; of the tornado, 105; ver- 
tical movement in, 83. 

Winter, temperature of, 50. 

Winter thaws, cause of, 127. 

Withered streams, 279. 



Yellow Sea, cause of color of, 152. 

Yellowstone Falls, 293. 

Yellowstone Park, geysers of, 387-389. 

Yellowstone Valley, 242, 268. 

Yosemite, 296, 398. 

Youth in river valleys, 266. 



«7 <4 



UNITED STATES, 

WITH BRIEFER MENTION OF FOREIGN MINERAL PRODUCTS, 

By RALPH S. TARR, B.S., P.G.S.A., 

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